Variable binary stars. Illustration depicting an RS Canum Venaticorum (RS CVn) binary star system. This is an extremely unusual class of star, where variation in brightness is due both to mutual eclipsing by the binary stars, and to intrinsic variability. Typically both members are G-type stars with extreme solar-like activity such as 'star-spots' (black) and flares. The intrinsic variability is due to large star spot groups rotating into view. The prototype binary system for this class is in the constellation Canes Venatici.