This week we investigate another object in the auxiliary data. We look at the known eclipsing binary star V2772Cyg, which has a distance of about 3kpc. The period of the binary is 2.77385d (2d 18h 34m 20+-10s). The phase folded light curve shows that the system consists of two very different stars, evidenced by the very different depths of the primary and secondary minima. The secondary minimum is, within the measurement uncertainties, exactly at a phase of 0.5, hence the orbit is most likely circular.
The primary minimum has a depth of 0.61mag, while during the secondary eclipse the brightness only drops by 0.11mag. Both eclipses last for 10% of the orbital period, i.e. they are about 6h 40m long. This is further evidence of a circular orbit.
What is also very evident is the ’rounding’ of the maxima between the eclipses. This is usually a sign of elliptical shapes of the stars in close systems. In other words tidal forces deform the stars from a spherical shape. One would hence expect the system to be the brightest at a phase of 0.25 and 0.75, as these are the times of a side-on view, where the observer sees the largest surface area. However, in this system the brightest point in the light curve occurs closer to the secondary eclipse (phase ~0.30-0.35 and ~0.65-0.70). It is not clear why this happens. But it would indicate that either the secondary star has a higher than average surface brightness on the side facing the the primary, or the primary has a higher than average surface brightness on the side facing away from the secondary.
A second slightly unusual feature of the light curve is the fact that the two maxima of the system are not of equal height. Indeed the maximum between primary and secondary eclipse is about 0.013mag (1.3%) fainter than the maximum between the secondary and primary eclipse. This is usually attributed to the O’Connell effect. This is either caused by spots on the stellar surfaces or evidence of matter streams between the two stars in the system – where the stream is not aligned with the line connecting the two stars, caused by Coriolis forces. Note that in short period binaries like this one, the orbits are not just circularised, but the stellar rotation periods are also locked, i.e. are the same as the orbital period.