Galaxy (eg de Jong et al 2010). On the other hand, if the periods are due to the first over-tone radial mode then these distances are less by $\sim$20 percent.

The two sources with the greatest distances (J030556 and J230507 at $\sim$30 kpc) have a Galactic longitude which appears to place them at a distance much further than the accepted limits of the spiral structure of the Milky Way (eg Churchwell et al 2009). Secondly, three sources (J030556, J120232, J230507) lie more than 10 kpc distant from the Galactic plane: this places them deep into the Galactic halo.

Followup Photometry
===================

To confirm the period of J0305, we obtained followup photometry of this source on 31 Dec 2010 using the Nordic Optical Telescope and ALFOSC. We used white light and an exposure time of 10 sec. The resulting light curve, binned into 120 sec bins, covers 3.8 hrs (Figure \[j0305\]). A clear modulation is present in the light curve. A Lomb Scargle power spectrum of the light curve indicates a period of 90 min. This compares with 83 min (Table 1) which was derived using the original INT light curve. Given the uncertainties in the period derived using each data-set, 