1995PASP..107..744C

Eight years of Ca II surface activity records from Mount Wilson Observatory measured for 12 bright G-K III stars have been analyzed in order to detect periodic variations attributable to rotation. We also present photometric V-band data for these stars from the Fairborn 0.25m Automatic Photometric Telescope (APT) that yielded a photometric period in one case and rms deviations from apparently constant brightness levels for the remaining 11 stars. The Ca II data yielded rotation periods for 10 out of 12 giant stars. We demonstrate that the photometric variability and non-variability of these stars can be predicted from their Rossby numbers calculated from our observed rotation periods and convective turnover times scaled up from the main sequence.