A BY Draconis star is a type of rotating variable dwarf star, of spectral type G, K or M, that shows quasiperiodic light changes, ranging from a few hundredths to 0.5 magnitude, with a period from a few hours up to 120 days. The variations are due to surface features, such as starspots, passing in and out of view, as the star spins on its axis. Some of these stars also show flares, similar to UV Ceti stars, in which case they belong to both types of variable star.

 

BY Draconis itself is a close binary consisting of a K6V dwarf and a M0V dwarf with an orbital period of 5.9 days and a mean separation of 0.05 astronomical units (AU). The M star exhibits flares that increase its luminosity periodically by a factor of 2 to 4, and also shows smaller variability keyed to its orbit and chromospheric activity.

 

A nearby example of a BY Draconis star is Gliese 581.