THE PERIOD-GRAVITY RELATION FOR RADIALLY PULSATING VARIABLE-STARS

It is pointed out that a log g vs log P relation exists that is almost universal among radially pulsating variable stars. A third variable m ay play a minor role, but it is not clear what this may be. Since log g can often be determined spectroscopically or photometrically or by o ther means, this relation can serve as a tool for testing the radial/n onradial nature of a star's pulsation, or conversely, if it is clear t hat the pulsation is radial, it can serve to determine a star's gravit y and hence help unravel the nature of the star itself. Additionally, if the gravity can be determined with adequate precision in favorable cases, the relation could decide between fundamental and first-harmoni c modes of radial pulsation. An application of this last kind to the l ong-period variable, Mira, suggests that Mira is pulsating in the firs t-harmonic mode if 2<P-0/P-1<21/2 for such stars. (C) 1995 American As tronomical Society.