This week we have an exotic EB, explained to us by Dr. Cole Johnston, where the primary star is a subdwarf which is the stripped helium-burning core of a star. The temperature of this star is so high that it illuminates the much cooler secondary star, causing the surface of the secondary star that is facing the primary to heat up and appear much brighter than the side that is facing away. This causes a dramatic increase in brightness approaching and receding from the secondary eclipse (the small dip at the top of the ‘wave’ in the above lightcurve). The two stars are so close together that they complete one orbit in just a few hours! The above light curve is phase folded to emphasise the brightening which is known as the ‘reflection effect’.
Studying these systems is important because these primary stars are thought to be the tracers of a very strange evolutionary path, whereby the entire hydrogen envelope of an evolving star is stripped away by some mechanism (probably by a binary or high mass planetary companion), just at the point were helium burning starts in the core of the star.