For one more week we are sticking with eclipsing binaries. In the field near the star P Cyg we find the object 2MASS J20180945+3818279. According to the SIMBAD database, this is an O or B type star. The Gaia parallax indicates that it is at a distance of about 5300 light years. The colour image from the Digitised Sky Survey (DSS) shows the object to be very blue, in agreement with the spectral type.

The source is a known eclipsing binary with a period of 1.661918 days. We have phase folded the HOYS data taken in the V-band filter with the known period. One can see that the primary (at phase=0 or phase=1) and secondary (at phase=0.5) eclipses are both of about the same depth. Both are about 0.3mag deep. The depth is the same in the other filters as well. This indicates that both stars are roughly of the same size and surface temperature. Thus, we most likely have here an equal mass binary. We do not see the orbit exactly edge on, as this would lead to a dimming of about 0.75mag. The depth measured indicates that during the eclipses about one quarter (25%) of one of the stars is eclipsed.

The secondary eclipse sits at exactly a phase of 0.5. This means the orbit of the two stars is circular. Given the very short period, the stars will be very close to each other. This leads to strong tidal forces causing orbital circularisation. Indeed, all short period binaries and exoplanets will over time have circular orbits.