Light curve of J0640+3856. The large dip in relative flux occurs when the more luminous sdO star disappears behind the red dwarf star, while the smaller dip in relative flux occurs when the sdO star passes in front of the less luminous red dwarf star. Derekas et al. (2015).
Spectroscopic and photometric observations of J0640+3856 show that the sdO star has 0.567 ± 0.138 times the Sun’s mass, 0.0955 ± 0.0077 times the Sun’s radius, 73.692 ± 11.819 times the Sun’s luminosity and a very hot surface temperature of 55,000 ± 3,000 K. As for the companion red dwarf star, it has 0.177 ± 0.051 times the Sun’s mass, 0.1985 ± 0.0159 times the Sun’s radius, 0.016 ± 0.004 times the Sun’s luminosity and a surface temperature of roughly 4,000 K. Both the sdO star and the red dwarf star are separated by a distance of only 1.25 times the Sun’s radius (i.e. a distance of 870,000 km).
Since the sdO star is orders of magnitude more luminous than its companion red dwarf star, the total brightness of the binary system falls nearly to zero when the sdO star vanishes from view behind the companion red dwarf star. The hot sdO irradiates the nearby companion red dwarf star so intensely that the “dayside” of the red dwarf star is heated to a temperature of about 22,500 K. The irradiation is so strong that the total brightness of the binary system actually raises and falls as the irradiated “dayside” of the red dwarf star rotates in and out of view. J0640+3856 appears to be a good analogue to planetary systems with hot-Jupiters in close-in orbits around Sun-like star.
Derekas et al. (2015), “A new sdO+dM binary with extreme eclipses and reflection effect”, arXiv:1505.06487 [astro-ph.SR]