Emission-line variability in quiescent flare stars.

Spectroscopic monitoring of five flare stars (UV Ceti, YZ CMi, BY Dra, YY Gem and BD +480 1958A) reveals that the stars exhibit marked decreases in the Balmer and Ca ii emission line strengths over intervals of one day or less. The variations can be explained by a non-uniform, patchy distribution of emitting regions on a rotating star. This is consistent with the starspot model from which in some other cases stellar rotation periods were inferred. Equatorial rotational velocities inferred are significantly greater than might be expected for ordinary stars at the same positions in the H-R diagram.