6781 d, i.e. 18.5 yr.

\[1\][![Times of primary eclipse provide an improved value for the orbital period $24\fd80091 \pm \rm 0\fd00044$. []{data-label=""}](timesprimary.pdf "fig:"){width="8.5cm"}]{}

\[1\][![image](LCIband.pdf){width="18cm"}]{}

\[1\][![Observed minus calculated times for primary eclipses using the orbital period given by the ephemeris of Eq.(1) and the best fit. Dashed area shows the 95% confidence band.[]{data-label=""}](OC.pdf "fig:"){width="8.5cm"}]{}

\[1\][![Eclipse magnitudes as a function of time. The lowest point of every colored data segment in the eclipses of Fig.2 is plotted.[]{data-label=""}](trends.pdf "fig:"){width="8.5cm"}]{}

\[1.1\][![The WWZ 2D power spectrum as a function of period and time.[]{data-label=""}](newWWZ.pdf "fig:"){width="8.5cm"}]{}

  band/Data-Base     N     $HJD'_{start}$   $HJD'_{end}$    Mag
  ---------------- ------ ---------------- -------------- --------
  I / OGLE-II       346      551.77073       1858.52147    13.213
  I / OGLE-III      795      2117.76494      4955.73490    13.221
  I / OGLE-IV       1465     5261.84891      7332.50545    13.213
  V/ OGLE-IV        118      5274.88451      7273.60682    15.038

  : Summary of photometric observations. The number of measurements, starting and ending times, and average magnitude are given. HJD’ = HJD-2450000. The uncertainty of a single measurement varies between 4 and 6 mmag. []{data-label="tab:example_table"}

\[1\][![image](segments.pdf){width="18cm"}]{}

\[1\][![Orbital light curves constructed with residuals of the fits shown in fig.7 for data spanning the time interval (HJD-245000) between 6334.9 and 7332.5. Every panel shows 50 consecutive data points. Time goes from left to right from upper left 