Radial Velocities and Spectral Characteristics of the Population II Cepheids m5 No. 42, m5 No. 84, and TW Capricorni.
Radial velocity-curves have been obtained for three population II cepheids with periods close to 27 days. Double lines have been observed for MS No. 42 and TW Capricorni, and it is shown that the velocity-curves of all population II cepheids of period greater than 15 days are probably discontinuous. The spectral changes of the three program stars are described and compared with standards on the Yerkes system of classification, the classical cepheid T Monocerotis, and other population II cepheids. These three stars have an early spectral type at maximum, A5-A6 I b-Il and get no later than F5. Hydrogen emission is present during increasing light but is not so strong as in W Virginis. Among the population II cepheids of period greater than 15 days there seems to be a correlation between earliest spectral type and shape of the light-curve. The general weakness of the hydrogen lines in absorption can be interpreted as meaning either that incipient emission is generally present or that the hydrogen-to-metal ratio is considerably higher than for standard stars.