The Semiregular Variable Stars of the RV Tauri and Related Classes.

Observations.-A spectroscopic survey of the F, G, and K semiregular variables was based on Mount Wilson spectrograms of thirty-eight stars. Physical characteristics.-Strong ionized lines indicate that many of the stars are as bright as, or brighter than, the long-period cepheids. The mean spectral type is about GO at maximum and K0 at minimum light. Titanium oxide bands occur in twenty-one stars and are seen on spectra as early as G4. The mean radial-velocity range during a cycle is 36 km/sec, but irregularities are so great that mean velocity-curves are impossible. The relation of velocity changes to light-variations is similar to that of the cepheids. Hydrogen emission is present in twenty-five stars at times of increasing light. The G band of CH becomes stronger with decreasing light and later spectral type. Subdivisions.-If the stars are divided according to radial velocity, the fast and slow groups are found to differ also in mean luminosity, spectral type, intensity of carbon bands, and distribution in the sky. The fast-moving group is undoubtedly of type II population, but the exact relationship of the slow group to Baade's populations is uncertain.