BY BY Draconis-type variables, which are emission-line dwarfs of dKe-dMe
spectral type showing quasiperiodic light changes with periods
from a fraction of a day to 120 days and amplitudes from several
hundredths to 0.5 mag in V. The light variability is caused by
axial rotation of a star with a variable degree of nonuniformity
of the surface brightness (spots) and chromospheric activity.
Some of these stars also show flares similar to those of UV Cet
stars, and in those cases they also belong to the latter type and
are simultaneously considered eruptive variables.