The photometric variability of solar-type stars. IV. Detection of rotational modulation among Hyades stars.

High-precision differential photometric observations of 32 Hyades stars, spectral type F4 V to K1 V, were obtained at Lowell Observatory over a five-month season during late 1982 and early 1983. Contemporaneous Ca II H+K emission flux measurements for 30 of these stars were made at Mount Wilson Observatory. Fifteen (possibly 18) of these stars, all later than type F7 V, were found to be photometrically variable. Highly significant correlations between continuum and Ca II emission variations were found in several cases. The photometric variations for nine of the stars were periodic. The authors have interpreted this as rotational modulation, since the measured periods agreed well with rotation periods predicted from measurements of the mean chromospheric emission fluxes for these stars.