Photometric variability in a sample of 187 G and K giants

We have used three automatic photoelectric telescopes to obtain photometric observations of 187 G, K, and (a few) MO field giants. We find low-amplitu de photometric variability on timescales of days to weeks on both sides of the coronal dividing line (CDL) in a total of 81 or 43% of the 187 giants. About one-third of the variables have amplitudes greater than 0.01 mag in V . In our sample the percentage of variable giants is a minimum for late-G s pectral classes and increases for earlier and later classes; all K5 and MO giants are variable. We also obtained high-resolution, red wavelength spect roscopic observations of 147 of the giants, which we used to determine spec tral classifications, v sin i values, and radial velocities. We acquired ad ditional high-resolution, blue wavelength spectra of 48 of the giants, whic h we used to determine chromospheric emission fluxes: We analyzed the photo metric and spectroscopic observations to identify the cause(s) of photometr ic variability in our sample of giants. We show that the light variations i n the vast majority of G and K giant variables cannot be due to rotation. F or giants on the cool side of the CDL, we find that the variability mechani sm is radial pulsation. Thus, the variability mechanism operating in M gian ts extends into the K giants up to about spectral class K2. On the hot side of the CDL, the variability mechanism is most likely nonradial, g-mode pul sation.