This week we have another look at one of the stars from the auxiliary dataset. It is of the star 2MASS J21083418+4656198. There is not much known about the star in the literature other than brightness and parallax. Hence, we know it is about 560pc away.
The light curve, however, shows that it is another eclipsing binary system. The period of the system is 3d 10h 6m 40s. The primary eclipse is about 0.3mag deep, while the secondary eclipse is 0.1mag deep. Hence clearly both stars differ in their properties, i.e. radius and temperature.
It is very apparent that the secondary and primary eclipses are not 0.5 in phase space apart. This indicates an elliptical orbit. From the shift one can estimate the minimum eccentricity of the system, roughly working out as 0.25. This is a minimum eccentricity as one usually does not know where in the orbit the position of the closest approach (periastron) of the two stars is, with respect to the observing direction.
However, in this case we can clearly see that the two eclipses are also of different length. The secondary eclipse is clearly longer than the primary one. This means the stars move faster during the primary eclipse, i.e. this one happens closer to periastron than the secondary eclipse. The combination of the two measurements of phase shift and duration change allows to determine the position of the periastron and the real eccentricity. The latter being 0.27-0.31 depending how you do the measurements. Note these are just rough estimates from the plot and not from a full detailed analysis yet.
It might be interesting to see if the ‘hump’ in the brightness between the two eclipses (0.007mag, i.e. 0.7 percent increase) is real, as it could hint to tidal deformation of the stars during the close encounter at periastron. Might be an interesting system for follow up studies including radial velocity measurements to determine the masses.