Binary incidence among the BY Draconis variables

Using new radial velocity data from coud spectrograms, we critically examine the incidence of spectroscopic binaries among the BY Draconis variables. Of 13 such stars with sufficient velocity data, eight are double-line spectroscopic binaries, one is a single-line binary, and four show no apparent velocity variation. With suitable con- straints imposed by the geometrical model for the origin of the BY Dra-type photomet- ric variations, we conclude that these latter four stars are truly single, and not in low-inclination orbits. It appears that a close companion is a sufficient, but not a necessary, condition for the occurrence of the BY Dra phenomenon. Instead, relatively rapid rotation [v(equatorial) >5 km ] is viewed as the ultimate cause of the BY Dra syndrome.