A STUDY OF VARIABILITY IN A SAMPLE OF G-GIANT AND K-GIANT

Eight years of Ca II surface activity records from Mount Wilson Observ atory measured for 12 bright G-K III stars have been analyzed in order to detect periodic variations attributable to rotation. We also prese nt photometric V-band data for these stars from the Fairborn 0.25-m Au tomated Photometric Telescope (APT) that yielded a photometric period in one case and rms deviations from apparently constant brightness lev els for the remaining 11 stars. The Ca II data yielded rotation period s for 10 out of 12 giant stars. We demonstrate that the photometric va riability and nonvariability of these stars can be predicted from thei r Rossby numbers calculated from our observed rotation periods and con vective turnover times scaled up from the main sequence.