Blue Large-Amplitude Pulsators and Other Short-Period Variable Stars in the
OGLE-IV Galactic Disk Fields
J. B o r o w i c z1, P. P i e t r u k o w i c z1, P. M r ó z1, I. S o s z y ń
s k i1,
A. U d a l s k i1, M. K. S z y m a ń s k i1, K. U l a c z y k2,
R. P o l e s k i1, S. K o z ł o w s k i1, J. S k o w r o n1,
D. M. S k o w r o n1, K. R y b i c k i1,3, P. I w a n e k1,
M. W r o n a1, and M. G r o m a d z k i1
1 Astronomical Observatory, University of Warsaw, Al. Ujazdowskie 4, 00-478
Warszawa, Poland

2 Department of Physics, University of Warwick, Coventry CV4 7AL, UK

3 Department of Particle Physics and Astrophysics, Weizmann Institute of
Science, Rehovot 76100, Israel

ABSTRACT

Blue Large-Amplitude Pulsators (BLAPs) form a mysterious class of variable
stars with typical periods of tens of minutes and amplitudes above 0.1 mag. In
this work, we present results of a variability search focused on timescales
shorter than 1 h, conducted in OGLE-IV Galactic disk fields containing about
1.1 billion stellar sources down to $I\approx 20$ mag. Twenty-five BLAPs have
been detected, 20 of which are new discoveries. Their periods range from 8.4
to 62.1 min. We have also found six new eclipsing binary systems with orbital
periods from 38.3 to 121.3 min and five short-period large-amplitude ($>0.17$
mag in the $I$-band) variable stars of unknown type.

Stars: oscillations (including pulsations) — Stars: variables: Blue Large-
Amplitude Pulsators — binaries: eclipsing

##  1 Introduction

Short-period variable stars are intriguing objects that exhibit brightness
variations on timescales from seconds to several hours. The variables
encompass various types of objects including single as well as binary stars,
such as $\delta$ Sct and SX Phe-type pulsating stars, ZZ Cet and V777 Her-type
oscillating white dwarfs, variable hot subdwarfs of V361 Hya and V1093 Her
type, rapidly oscillating Ap (roAp) stars, Blue Large-Amplitude Pulsators
(BLAPs), AM CVn-type binary systems, among others. Numerous short-period
variables have been found over the last few years, including pulsating hot
subdwarfs (e.g., Kupfer et al. 2021), pulsating white dwarfs (e.g., Guidry et
al. 2021), compact binary systems (e.g., Burdge et al. 2019, 2020, Ren et al.
2023), and other low-amplitude stellar sources (e.g., Toma et al. 2016, Lin et
al. 2023b).

In our work, we focus on the detection and classification of short-period
variable stars in the southern part of the Milky Way’s stripe observed by the
Optical Gravitational Lensing Experiment (OGLE). Although, the OGLE disk
fields have been surveyed for variable objects such as Cepheids (Udalski et
al. 2018), RR Lyr (Soszyński et al. 2019), and Mira-type stars (Iwanek et al.
2022), those studies focused on longer periods (roughly from hours to months).
In the collection of Galactic disk $\delta$ Sct-type stars (Soszyński et al.
2021), there are seven entries with periods below 60 min, with the shortest
reported value of 50.2 min. Our goal is an extensive search of the previously
unexplored variability range ($<1$ h) to find the mysterious BLAPs and compact
binary systems.

BLAPs are a recently discovered class of pulsating stars (Pietrukowicz et al.
2017). They were identified thanks to regular, high-precision photometric
observations of hundreds of millions of stars by the OGLE survey (Udalski et
al. 2015). BLAPs can be characterized by significant amplitudes of brightness
variations, typically 0.1–0.4 mag in optical bands, at very short periods, in
the range 3–75 min (Pietrukowicz et al. 2023, submitted). The variability in
the stars is attributed to fundamental-mode radial pulsations. Light curve
shapes are very similar to those of Cepheids and RR Lyr-type stars, but at
much higher temperatures. BLAPs have effective temperatures of $T_{\rm{eff}}$
from about 25 000 K to 34 000 K and surface gravities of $\log g$ from about
4.2 to 5.7 dex. They exhibit a reduced hydrogen content in their atmospheres,
compared to main-sequence stars. Such a set of atypical properties makes it
difficult to assign specific evolutionary stage to these stars.

It has been proposed that BLAPs are low-mass stars in the pre-white dwarf
stage, during which they produce energy in residual hydrogen burning over the
degenerate helium core (Romero et al. 2018). Other models indicate that the
stars could be in the phase of helium burning in their cores (Byrne and
Jeffery 2018, Wu and Li 2018). A third possibility is that BLAPs are shell
helium-burning stars with the CO core (Lin et al. 2023a). The stability of
pulsation periods ($|1/P\times\Delta P/\Delta t|<10^{-6}$ yr-1) suggests that
these objects evolve on the nuclear timescale. Known BLAPs reside in the Milky
Way’s bulge and disk. They seem to be absent in the Galactic halo, globular
clusters, and Magellanic Clouds (Pietrukowicz 2018). Proposed mechanisms
describing the evolutionary history of BLAPs include interactive binary
evolution (common envelope phase and/or Roche lobe overflow, Byrne et al.
2021), a close flyby of the supermassive black hole in the center of the
Galaxy (Pietrukowicz et al. 2017), and a result (as surviving companions) of
single-degenerate Type Ia supernovae (Meng et al. 2020). BLAPs are of
particular interest because their pulsations can provide insight into the
structure and physical processes in hot (Z-bump) pulsators (Byrne and Jeffery
2018).

So far, over 60 BLAPs have been found, mainly by large-scale surveys. After
the discovery of the first 14 pulsators by OGLE (Pietrukowicz et al. 2017),
four ultra short-period (3–8 min) BLAPs, characterized by higher gravities
(hence named HG-BLAPs), were found in the Zwicky Transient Facility (ZTF;
Bellm et al. 2019) data by Kupfer et al. (2019). Recently, 22 candidates for
BLAPs from ZTF and four candidates from the OmegaWhite survey (Macfarlane et
al. 2015) were proposed by McWhirter and Lam (2022) and Ramsay et al. (2022),
respectively. At the moment of writing, another 23 objects from the OGLE
Galactic bulge fields can be added to the list of known pulsators
(Pietrukowicz et al. 2023, submitted). Finally, first two BLAPs residing in
binary systems have been lately reported. Using data from the TESS satellite
Pigulski et al. (2022) discovered a pulsator orbiting a B-type main-sequence
star HD 133729 every 23.08 days. Based on observations collected with the
Tsinghua University-Ma Huateng Telescopes for Survey (TMTS; Lin et al. 2022),
Lin et al. (2023a) suggested that object TMTS-BLAP-1 is in a 1576-day binary.

This paper is organized as follows. In Sec. 2, we briefly present the OGLE
survey and the collected data. In Sec. 3, we describe the data analysis. The
detected short-period variables are listed in Sec. 4. We summarize our results
in Sec. 5.

##  2 Observations

The photometric data used for the variability search comes from the fourth
phase of the OGLE project (OGLE-IV) which started in March 2010. OGLE has been
at the forefront of large-scale photometric sky surveys for over 30 years. The
project monitors the southern sky using the 1.3-m Warsaw telescope located at
the Las Campanas Observatory in Chile***The observatory is operated by the
Carnegie Institution for Science.. The most frequently observed regions are
the Galactic bulge and Magellanic Clouds. Shallow and less frequent
observations of the Galactic disk are conducted since 2013. Currently, the
total area observed by the project reaches almost 3600 square degrees.
Overall, there are 1338 OGLE-IV disk fields with negative Galactic longitude
(GD fields), labeled from GD1063 to GD2475 (with some gaps in numbering).
Slightly different nomenclature is followed for 354 disk fields with positive
Galactic longitudes. These span from DG1000 to DG1364 (no field numbers
between DG1288 and DG1300). The OGLE-IV images are taken using a 256-megapixel
mosaic camera with a pixel size of
$0\makebox[0.0pt][l]{.}{{}^{\prime\prime}}26$ and a 1.4-square-degree field of
view, consisting of 32 single 2K$\times$4K CCD detectors. Two filters are
used, $V$ and $I$, with the majority of observations taken in the $I$-band.
The $I$ filter closely resembles the standard Kron-Cousins filter system,
whereas the $V$ filter differs from the standard system to a greater extent.
Fields in the Galactic disk are monitored with exposure times of 25 s, while
for fields in the inner bulge, it is 100 s. Typically for the GD fields,
around 100–200 regular $I$-band measurements and several $V$-band measurements
have been obtained. For the DG fields, 10–130 $I$-band observations were
taken, but no time-series $V$-band photometry is available. The brightness
range in $I$ is between 10.5 and 20 mag. In Fig. 1, we show accuracy of the
$I$-band observations for a selected GD field. In total, there are
approximately 1.1 billion stellar sources over the analyzed area of about 2400
square degrees of the Galactic disk. More details on the OGLE-IV survey can be
found in Udalski et al. (2015).

Fig. 1. Accuracy of the I-band photometry in one of the OGLE-IV Galactic disk
subfields (GD1915.23).

From January to March 2020, additional 3-hour high-cadence observing runs were
carried out for selected dense GD fields, resulting in about 100 additional
$I$-band measurements per field. This was done for 31 pairs of fields,
observed alternately. Shortly after, on 18 March 2020, observations were
suspended due to the COVID-19 pandemic. OGLE resumed regular monitoring of the
sky on 12 August 2022.

##  3 Data Analysis

Our search for short-period variable stars was conducted with the $I$-band
time-series data. For the analysis, we selected sources brighter than $I=20$
mag and with more than 50 measurements. Several DG fields were not included
due to small number of observations. The first step was the search for
periodicity in all sources using the
FNPEAKS111http://helas.astro.uni.wroc.pl/deliverables.php?active=fnpeaks&lang=en
program, which performs a discrete Fourier transform on unevenly spaced data.
We probed a frequency range of $0<f<1000$ cycles per day ($P~{}>~{}1.44$ min).
Subsequently, only objects with a significant signal-to-noise ratio were
selected. The variability signal varied from field to field. The 3-hour high-
cadence sequences were searched for periodic signals separately.

In the next step, all objects with periods shorter than 0.05 d were visually
inspected. We cleaned our lists of candidate variables from artifacts (objects
with fake variability). Obvious outlying points from the light curves of real
variables were removed. About 1100 variable stars with periods below 1 h were
detected. Stars with amplitudes higher than 0.1 mag were a subject of further
inspection. We obtained a set of BLAP candidates based on morphology of the
light curves. Variables with low amplitudes were not classified due to
difficulties in assigning them to a definitive type. The majority of these
objects are likely low-amplitude $\delta$ Sct stars, similar to those found in
the Galactic bulge and recently confirmed spectroscopically (Pietrukowicz et
al. 2022). Variables exhibiting peculiar light curve shapes after doubling the
period were also noted.

We verified the positions of the objects in color-magnitude diagrams (CMDs).
The diagrams were constructed for stars from the corresponding OGLE-IV
subfields. For objects without $V$-band measurements, we downloaded available
data from other sources. In the case of stars from DG fields, we downloaded
time-series data in $r$ and $g$-bands from the ZTF survey. For objects from GD
fields for which there were no available $V$-band measurements, we used data
from the DECam Plane Survey (Schlafly et al. 2018). The final list of BLAPs
was created based on the light curve morphology, period, amplitude, and
position on the CMD. Moments of the observations were transformed from
heliocentric Julian date (HJD) to barycentric Julian date (BJDTDB) and the
periods were fine-tuned using the TATRY code (Schwarzenberg-Czerny 1996). All
of the detected objects were checked for additional periodicities.

##  4 Results

The main outcome of this paper are the lists of BLAPs, close eclipsing binary
stars, and other short-period variables identified in the OGLE-IV Galactic
disk fields. Here, we present phase-folded light curves, along with CMDs, for
all the discovered objects. The time-series data are publicly available in the
OGLE Collection of Variable Stars (OCVS):

https://www.astrouw.edu.pl/ogle/ogle4/OCVS/

In the case of BLAPs, we follow the nomenclature used for previously
classified variable stars in the OGLE fields. We continue numbering the new
objects in fasion OGLE-BLAP-NNN, starting from NNN=038.

###  4.1 BLAPs

The majority of detected short-period variables are of BLAP type. Among the 25
identified BLAPs, 20 objects are new discoveries. Of the five known BLAPs are
the prototype and four objects from McWhirter and Lam (2021). We note that
nine of the 25 detected BLAPs were classified as variable sources in Gaia DR3
(Eyer et al. 2023), but only three of them belong to the ”S” class (short
timescale objects). The remaining six stars were classified by Gaia to either
the ”DSCT—GDOR—SXPHE” class (four stars), or to the ”ECL” class (one object),
or to the ”RS” class (RS CVn-type, one object). Only for the new variable
OGLE-BLAP-039 from the ”S” class, Gaia provides the correct period.

Observational parameters, such as coordinates, pulsation periods, magnitudes,
and amplitudes of the detected BLAPs are listed in Table 1. Parallaxes
$\varpi$ are taken from Gaia EDR3 catalog (Gaia Collaboration 2021). In Fig.
2, we present the period–amplitude diagram with positions of previously
discovered BLAPs by OGLE and BLAPs detected in this work. For comparison, the
figure also includes single-mode $\delta$ Sct-type stars from the OCVS in the
Galactic disk (Soszyński et al. 2021). There are no evident differences in
location in the period–amplitude diagram between the objects observed toward
the disk and bulge fields. Phase-folded $I$-band light curves and $V-I$ vs.
$I$ CMDs for 18 BLAPs detected in the OGLE-IV Galactic disk fields with
negative longitudes (GD fields) are shown in Figs. 3–6. For seven pulsators
from the OGLE-IV disk fields with positive Galactic longitudes (DG fields),
for which no $V$-band measurements are available, the light curves and CMDs
based on ZTF data are provided next to the OGLE $I$-band light curves (Figs. 7
and 8). All the OGLE light curves are based on $I$-band measurements, whereas
to construct ZTF light curves, we used either $g$ or $r$-band data. In the
case of the CMD for object OGLE-BLAP-045, we used the DECam Plane Survey data
in the $g$ and $i$ filters.

Table 1: Photometric properties of detected BLAPs

Name | RA (J2000) | Dec (J2000) | $l$ | $b$ | $\varpi$ | $\sigma_{\varpi}$ | $\textit{P}_{\rm puls}$ | $\sigma_{P}$ | $I$ | $V-I$ | $A_{I}$ | Other name
---|---|---|---|---|---|---|---|---|---|---|---|---
| [h:m:s] |  [${}^{\circ}:^{\prime}:^{\prime\prime}$] |  [∘] |  [∘] | [mas] | [mas] | [min] | [$10^{-7}$ min] | [mag] | [mag] | [mag] | [Ref.]
OGLE-BLAP-001 | 10:41:48.77 |  $-$61:25:08.5 | 288.06356 |  $-$2.34712 | 0.14 | 0.07 | 28.2549636 | 33 | 17.217 | 0.335 | 0.194 | [1]
OGLE-BLAP-038 | 07:24:10.65 |  $-$28:10:11.8 | 241.61485 |  $-$5.94461 | — | — | 11.2601526 | 60 | 18.822 | 0.844 | 0.413 | —
OGLE-BLAP-039 | 11:00:12.91 |  $-$61:13:44.9 | 289.95454 |  $-$1.19158 | 0.21 | 0.03 | 31.9651131 | 19 | 15.835 | 0.285 | 0.242 | —
OGLE-BLAP-040 | 11:01:57.46 |  $-$65:05:53.2 | 291.73311 |  $-$4.63443 | 0.26 | 0.02 | 31.4607770 | 24 | 15.178 | 0.337 | 0.243 | —
OGLE-BLAP-041 | 11:24:36.52 |  $-$58:45:58.5 | 291.86639 | 2.22748 | 0.25 | 0.03 | 53.5405245 | 115 | 15.371 | 0.390 | 0.191 | —
OGLE-BLAP-042 | 11:55:36.75 |  $-$64:26:53.4 | 296.95618 |  $-$2.22606 | 0.33 | 0.09 | 62.0505860 | 96 | 16.993 | 1.046 | 0.147 | —
OGLE-BLAP-043 | 12:04:34.02 |  $-$62:12:29.7 | 297.49826 | 0.16800 | 0.20 | 0.22 | 19.0642504 | 14 | 18.125 | 1.505 | 0.342 | —
OGLE-BLAP-044 | 12:29:31.78 |  $-$60:48:46.1 | 300.26251 | 1.94534 | 0.54 | 0.03 | 8.4216381 | 6 | 15.713 | 0.189 | 0.211 | —
OGLE-BLAP-045 | 13:04:41.30 |  $-$62:21:23.4 | 304.46825 | 0.47572 | 0.23 | 0.07 | 41.3305960 | 109 | 16.457 | — | 0.191 | —
OGLE-BLAP-046 | 13:08:15.31 |  $-$64:39:09.3 | 304.73142 |  $-$1.83961 | 0.04 | 0.08 | 39.4729321 | 91 | 17.065 | 0.782 | 0.207 | —
OGLE-BLAP-047 | 15:25:53.96 |  $-$52:04:54.8 | 325.53868 | 3.87749 | 0.12 | 0.06 | 31.2450830 | 32 | 16.032 | 0.664 | 0.239 | —
OGLE-BLAP-048 | 15:45:16.20 |  $-$59:21:46.0 | 323.49358 |  $-$3.61890 | 0.38 | 0.03 | 28.8555579 | 59 | 15.041 | 0.276 | 0.296 | —
OGLE-BLAP-049 | 16:00:21.31 |  $-$57:49:26.4 | 325.99337 |  $-$3.67615 | 0.31 | 0.08 | 16.4015624 | 7 | 16.597 | 0.367 | 0.375 | —
OGLE-BLAP-050 | 16:21:04.56 |  $-$57:33:03.0 | 328.21106 |  $-$5.36464 | 0.27 | 0.03 | 22.9352027 | 33 | 15.516 | 0.106 | 0.185 | —
OGLE-BLAP-051 | 16:25:12.57 |  $-$47:39:58.4 | 335.68801 | 1.14111 | 0.37 | 0.06 | 45.4160893 | 40 | 15.704 | 1.046 | 0.257 | —
OGLE-BLAP-052 | 16:31:34.60 |  $-$53:31:30.8 | 332.15103 |  $-$3.63874 | 0.22 | 0.03 | 34.9622342 | 79 | 15.284 | 0.267 | 0.194 | —
OGLE-BLAP-053 | 16:31:35.55 |  $-$51:35:22.9 | 333.56826 |  $-$2.31842 | — | — | 25.2601420 | 76 | 18.323 | 1.180 | 0.330 | —
OGLE-BLAP-054 | 17:17:15.31 |  $-$49:09:47.8 | 340.03800 |  $-$6.45954 | — | — | 11.3324455 | 71 | 17.402 | 0.227 | 0.335 | —
OGLE-BLAP-055 | 18:37:34.76 |  $-$04:22:27.0 | 27.45919 | 1.08009 | 0.17 | 0.26 | 35.3945271 | 161 | 17.691 | — | 0.330 | —
OGLE-BLAP-056 | 18:46:49.17 |  $-$10:44:27.7 | 22.83623 |  $-$3.86251 | 0.29 | 0.16 | 11.6725445 | 17 | 17.845 | — | 0.176 | —
OGLE-BLAP-057 | 18:47:45.81 |  $-$10:26:06.0 | 23.21496 |  $-$3.93102 | 0.13 | 0.05 | 54.6530265 | 114 | 15.752 | — | 0.112 |  ZGP-BLAP-05[2]
OGLE-BLAP-058 | 18:52:45.11 |  $-$05:10:47.2 | 28.46814 |  $-$2.65077 | 0.22 | 0.11 | 35.8390155 | 29 | 16.647 | — | 0.314 |  ZGP-BLAP-06[2]
OGLE-BLAP-059 | 18:54:01.82 |  $-$06:33:22.2 | 27.38398 |  $-$3.55829 | 0.13 | 0.11 | 33.5823049 | 91 | 17.113 | — | 0.296 | —
OGLE-BLAP-060 | 19:03:29.08 |  $-$00:05:02.0 | 34.23148 |  $-$2.71840 | 0.16 | 0.09 | 44.6277164 | 118 | 16.836 | — | 0.109 |  ZGP-BLAP-07[2]
OGLE-BLAP-061 | 19:13:08.78 | 12:04:51.5 | 46.13835 | 0.75200 | 0.44 | 0.08 | 35.1375078 | 107 | 16.347 | — | 0.232 |  ZGP-BLAP-08[2]

References: [1] Pietrukowicz et al. (2017), [2] McWhirter and Lam (2021)

Fig. 2. Period–amplitude diagram for all 61 BLAPs detected in the OGLE fields.
Twenty-five stars from the Galactic disk fields (including the prototype
object OGLE-BLAP-001) are marked with blue empty squares. The original 13
BLAPs (Pietrukowicz et al. 2017) and 23 additional BLAPs recently found in the
Galactic bulge fields (Pietrukowicz et al. 2023, submitted) are marked with
blue filled triangles. Positions of $\delta$ Sct stars from the OGLE-IV
Galactic disk are represented with gray points.

Fig. 3. Phase-folded I-band light curves (left panels) and color-magnitude
diagrams (right panels) for BLAPs detected in the OGLE-IV Galactic disk
fields. The diagrams were constructed for stars from the corresponding OGLE-IV
subfield. Positions of the BLAPs are marked with blue dots. A significant
period change (as the light curve shift) can be noted in object OGLE-BLAP-039.

Fig. 4. Same as Fig. 3 for another five BLAPs. Light curve blur in object
OGLE-BLAP-044 is caused by significant period change. The color-magnitude
diagram for OGLE-BLAP-045 is based on data from the DECam Plane Survey.

Fig. 5. Same as Figs. 3–4 for another five BLAPs.

Fig. 6. Same as Figs. 3–5 for the remaining three BLAPs. Points that seem to
be outliers in the light curve of OGLE-BLAP-054 result from a large period
change.

In our sample, star OGLE-BLAP-044 has the shortest pulsation period of 8.42
min, which is the shortest value among all BLAPs detected in the OGLE data so
far. The longest period of 62.05 min is observed in OGLE-BLAP-042, which is
the second longest known BLAP after OGLE-BLAP-022 with $P=74.05$ min
(Pietrukowicz et al. 2023, submitted). For some of the objects, period changes
are clearly noticeable. This can be seen in the case of OGLE-BLAP-039 (for
which the majority of observations were taken over three nights), OGLE-
BLAP-044, and OGLE-BLAP-055. $I$-band amplitudes in our BLAP sample are in the
range of 0.109–0.413 mag. In the light curves of some objects, a bump or dip
around the maximum is present, notably in OGLE-BLAP-040, 045, 047, 057, and
060. We noticed that some objects have amplitudes and light curve shapes very
similar to each other despite slightly different periods. This refers to OGLE-
BLAP-057 ($P=54.65$ min, $A_{I}=0.112$ mag) and OGLE-BLAP-060 ($P=44.63$ min,
$A_{I}=0.109$ mag), if one compares the rich ZTF-$r$ light curves. Object
OGLE-BLAP-047 ($P=31.24$ min, $A_{I}=0.239$ mag) is an almost perfect analog
of star OGLE-BLAP-009 ($P=31.93$ min, $A_{I}=0.242$ mag) observed toward the
Galactic bulge (Pietrukowicz et al. 2017). There is an unusual scatter around
the maximum light in object OGLE-BLAP-040. This object is present in two
overlapping OGLE-IV fields. The aforementioned effect is seen in both light
curves implying that it is a real intrinsic feature of the star. The new BLAPs
show only one periodicity. Based on Gaia EDR3 parallaxes (Gaia collaboration
et al. 2021), we can conclude that all the reported here BLAPs are rather
distant stars located at several kpc.

Fig. 7. Phase-folded light curves (left columns) and color-magnitude diagrams
(right columns) for BLAPs detected in OGLE-IV Galactic disk fields with
positive longitude (DG fields). The light curves plotted in red are based on
ZTF r-band measurements, whereas blue light curves use OGLE-IV $I$-band data.
The diagrams are constructed for stars located within a 10${}^{\prime}\times
10^{\prime}$ box centered on the target BLAPs using only ZTF data.

Fig. 8. Same as Fig. 7 for the remaining stars from OGLE-IV DG fields. For the
CMDs g and r-band data from ZTF were used. Light curves of objects OGLE-
BLAP-059 and OGLE-BLAP-061 plotted in red are based on ZTF $g$-band data.

###  4.2 Eclipsing Binary Systems

Our analysis of the OGLE-IV Galactic disk data led to the identification of
six short-period eclipsing binary systems. None of them were known earlier. In
Fig. 9, we present the CMD next to the light curve folded with the orbital
period for each of the systems, while in Table 2, we provide their photometric
properties. All the binaries but object GD1548.32.5823 have the orbital
periods $P_{\rm orb}<2$ h. The shortest detected system, GD1468.32.3751, has
$P_{\rm orb}=38.31$ min. This is the only system with the orbital period below
1 h. Objects GD1200.31.11089 and GD1548.32.5823 seem to be contact systems,
while the remaining four objects are likely semi-detached systems. Both
candidates for contact systems have the orbital periods shorter than the
shortest-period object of this type found in the OGLE data so far (OGLE-BLG-
ECL-000066, $P=2.36$ h; Soszyński et al. 2015). Due to the lack of $V$-band
measurements, the CMD for GD1200.31.11089 is constructed based on data from
the DECam Plane Survey. We found no X-ray counterparts to the discovered
systems.

Fig. 9. Phase-folded I-band light curves (left panels) and CMDs (right panels)
for eclipsing binary systems found in the OGLE-IV Galactic disk fields. The
CMD for GD1200.31.11089 is based on data from the DECam Plane Survey.

###  4.3 Other Short-Period Variable Stars

In addition to the previous objects, six short-period large-amplitude variable
stars were detected (see Table 3). The variables were noticed thanks to
peculiar shapes of the light curves, often after doubling the period. All of
the stars have period below 2 h. Phase-folded curves and CMDs are presented in
Fig. 10. It is challenging to indicate whether their variability is due to
binarity or rotational effect, in particular in the case of objects
GD1537.05.11659 and GD2077.07.161. We note that star GD1670.31.3740 has an
X-ray counterpart detected by ROSAT satellite (Boller et al. 2016). The source
2RXS J071305.7-084519 is separated by
$5\makebox[0.0pt][l]{.}{{}^{\prime\prime}}6$ from the star, which is
comparable to the spatial resolution of ROSAT. The detection of X-rays
indicates accretion processes and thus binary nature of the source. This
object as the only one in the group was found to be variable in Gaia DR3 and
classified as a cataclysmic variable (to the ”CV” class). Our light curve
exhibits a long-term trend, which we subtracted before phasing. The period of
114.61 min, not half of this value, seems to be correct for this type of
binary systems. In Fig. 11, we show the time-domain and phase-folded light
curve before detrending. Its full amplitude reaches 2.1 mag in $I$. Star
GD2117.17.7515 has a very odd light curve shape with an $I$-band amplitude of
0.483 mag. From Gaia EDR3 astrometry, we know that this is a relatively close
object ($215\pm 9$ pc, Bailer-Jones et al. 2021). The period of 90.75 min and
blue color of the object indicate that this is probably another cataclysmic
variable. Out of the 12 detected eclipsing binaries and other short-period
variables, five objects are located at relatively close distances, below 0.5
kpc. The remaining seven stars are located farther than 1 kpc.

Table 2: Photometric properties of detected eclipsing binary systems

OGLE-IV ID | RA (J2000) | Dec (J2000) | $l$ | $b$ | $\varpi$ | $\sigma_{\varpi}$ | $\textit{P}_{\rm orb}$ | $\sigma_{P}$ | $I$ | $V-I$ | $A_{I}$ | Other name
---|---|---|---|---|---|---|---|---|---|---|---|---
| [h:m:s] |  [${}^{\circ}:^{\prime}:^{\prime\prime}$] |  [∘] |  [∘] | [mas] | [mas] | [min] | [$10^{-7}$ min] | [mag] | [mag] | [mag] | [Ref.]
GD1665.11.7920 | 07:19:41.47 |  $-$19:53:47.3 | 233.79233 |  $-$2.99652 | 3.18 | 0.32 | 91.0734004 | 133 | 18.831 | 0.885 | 1.133 | —
GD1548.32.5823 | 08:08:39.16 |  $-$32:50:07.9 | 250.46588 |  $-$0.00261 | 0.23 | 0.07 | 121.3136403 | 215 | 17.055 | 0.602 | 0.341 | —
GD1468.32.3751 | 09:01:00.75 |  $-$48:12:45.4 | 268.68136 |  $-$1.27610 | 0.51 | 0.17 | 38.3057050 | 80 | 18.004 | 1.340 | 0.222 | —
GD1377.20.269 | 10:35:38.92 |  $-$54:47:29.8 | 284.12262 | 3.04366 | 0.30 | 0.03 | 97.7832499 | 92 | 16.066 | 0.047 | 0.368 | —
GD1200.31.11089 | 15:10:44.42 |  $-$58:01:22.2 | 320.53575 | 0.00644 | 3.22 | 1.02 | 116.5218887 | 464 | 19.005 | — | 0.543 | —
GD1118.10.37641 | 16:38:39.19 |  $-$51:47:40.5 | 334.15607 |  $-$3.26420 | 0.50 | 0.09 | 82.9840853 | 207 | 17.073 | 0.179 | 0.338 | —

Table 3: Photometric properties of other short-period variable stars

OGLE-IV ID | RA (J2000) | Dec (J2000) | $l$ | $b$ | $\varpi$ | $\sigma_{\varpi}$ | P | $\sigma_{P}$ | $I$ | $V-I$ | $A_{I}$ | Other name
---|---|---|---|---|---|---|---|---|---|---|---|---
| [h:m:s] |  [${}^{\circ}:^{\prime}:^{\prime\prime}$] |  [∘] |  [∘] | [mas] | [mas] | [min] | [$10^{-7}$ min] | [mag] | [mag] | [mag] | [Ref.]
GD1670.31.3740 | 07:13:06.14 |  $-$08:45:22.1 | 223.18737 | 0.77988 | 2.46 | 0.08 | 114.6144716 | 813 | 16.720 | 0.893 | 2.149 |  2RXS J071305.7-084519[1]
GD1537.05.11659 | 08:15:30.78 |  $-$34:21:23.4 | 252.52163 | 0.35499 | 0.51 | 0.05 | 73.7073948 | 95 | 16.845 |  $-$0.032 | 0.211 | —
GD2117.17.7515 | 08:22:39.46 |  $-$42:58:52.7 | 260.43536 |  $-$3.35434 | 4.68 | 0.24 | 90.7473704 | 1059 | 18.320 | 1.247 | 0.483 | —
GD2077.07.161 | 09:18:31.83 |  $-$60:23:22.6 | 279.39515 |  $-$7.65360 | 0.45 | 0.04 | 101.1611974 | 637 | 16.063 |  $-$0.008 | 0.206 | —
GD2011.25.1811 | 12:34:18.86 |  $-$68:14:56.5 | 301.33966 |  $-$5.43022 | 0.57 | 0.16 | 88.0693069 | 1054 | 16.995 | 1.319 | 0.171 | —
GD1072.09.419 | 17:09:09.45 |  $-$44:04:03.9 | 343.37511 |  $-$2.35751 | 6.76 | 0.13 | 105.9106750 | 660 | 16.995 | 1.207 | 0.201 | —

References: [1] Boller et al. (2016)

Fig. 10. Other short-period large-amplitude variable objects found in the
OGLE-IV Galactic disk fields. The light curve of object GD1670.31.3740 was
phase-folded after subtraction of a long-term trend shown in Fig. 11.

Fig. 11. Time-domain (left panel) and phase-folded (right panel) $I$-band
light curve of object GD1670.31.3740 exhibiting a long-term trend. Note a huge
amplitude of the brightness changes in this star.

##  5 Summary

This study focused on the search for short-period ($<1$ h) variable stars in
the OGLE-IV Galactic disk fields covering almost 2400 square degrees, with a
specific emphasis on the detection of BLAPs and compact binary systems. The
monitoring of about one billion stellar sources allowed the detection of 37
large-amplitude ($>0.1$ mag) variable stars. We identified 25 BLAPs, 20 of
which are new findings. Altogether with the very recent detection of 23
objects toward the inner Galactic bulge (Pietrukowicz et al. 2023, submitted),
our search increased the number of BLAPs to over 80. We reported the detection
of the shortest-period known BLAP in the OGLE area, OGLE-BLAP-044 with
$P=8.42$ min. In addition to the BLAPs, we identified six short-period
eclipsing binary systems with orbital periods in the range 38.3–121.3 min. We
informed about the detection of additional six short-period large-amplitude
variable stars. Two of the stars are likely cataclysmic variables. Follow-up
spectroscopic observations would allow their final classification.

Acknowledgments. We thank all the OGLE observers for their contribution to the
collection of the photometric data over the decades. This work has been funded
by the National Science Centre, Poland, grant no. 2022/45/B/ST9/00243 to I.S.
For the purpose of Open Access, the author has applied a CC-BY public
copyright licence to any Author Accepted Manuscript (AAM) version arising from
this submission. We used data from the European Space Agency (ESA) mission
Gaia, processed by the Gaia Data Processing and Analysis Consortium (DPAC).
Funding for the DPAC has been provided by national institutions, in particular
the institutions participating in the Gaia Multilateral Agreement.

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