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 on timescales equivalent to the star's rotational period, typically from a fraction of a day to several months. Oddly enough, Procyon the 8th brightest night-time star which is an F5 sub-giant or dwarf has also been classified as a BY Draconis variable.
Some of these stars may exhibit flares, resulting in additional variations of the UV Ceti type. Likewise, the spectra of BY Dra 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.
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