BY Draconis variables are main-sequence variable stars of late spectral types, usually K or M. The name comes from the archetype for this category of variable star system, BY Draconis. They exhibit variations in their luminosity due to rotation of the star coupled with star spots, and other chromospheric activity. Resultant brightness fluctuations are generally less than 0.5 magnitudes. Light curves from BY Draconis variables are quasiperiodic. The period is close to the star's mean rotational rate. The light curve is irregular over the duration of the period and the light curve changes slightly in shape from one period to the next. For the star BY Draconis the shape of the light curve over a period remained similar for a month.
Nearby K and M stars that are BY Draconis variables include Barnard's Star, Kapteyn's Star, 61 Cygni, Ross 248, Lacaille 8760, Lalande 21185 and Luyten 726-8. Ross 248 is the first discovered BY Draconis variable, the variability having been identified by Gerald E. Kron on 1950. The variability of BY Draconis itself was discovered in 1966 and studied in detail by Sugainov over the period 1973-1976.
Procyon, which is much brighter than the Sun and has a spectral type of F5 IV/V, is also claimed to be a BY Draconis variable. Procyon is unusual in two ways: it is moving off the main sequence into the subgiant phase, and it is midway between PP burning and CNO burning, which means that it is intermediate between having a convective and non-convective outer layer. Either reason could explain the larger than normal star spots.
Some of these stars may exhibit flares, resulting in additional variations of the UV Ceti type. Likewise, the spectra of BY Draconjs variables (particularly in their H and K lines) are similar to RS CVn stars, which are another class of variable stars that have active chromospheres.