A type of rotating variable star; abbr. BY. The variations are caused by non-uniform surface brightness and show a quasi-periodicity related to the rotation period. The latter ranges from a few hours to about 120 days, and the amplitude from a few hundredths to 0.5 mag. BY Draconis stars are K–M dwarfs. The non-uniform brightness is caused by starspots and chromospheric activity, similar to that on the Sun. Many BY Draconis stars are members of short-period spectroscopic binaries. BY Draconis itself consists of a pair of K dwarfs with an orbital period of 5.98 days; the variable component is BY Dra A. The pair varies from about magnitude 8.0 to 8.5 as BY Dra A rotates every 3.8 days. A BY Draconis star may simultaneously be a flare star.