The Radial Velocity of RR Lyrae.

Measurements of radial velocity were made on spectrograms secured between June 3 and July 15, 1947. The velocity-curve was found to oscillate in time with a period of approximately 75 cycles, or 42 days. On June 5 and again on July 17 the velocity-curve was retarded by approximately O024P, this being the maximum retardation observed. Halfway between, or approximately on June 26, the velocity- curve was advanced by about O.024P, this being the maximum advance observed. The principal period, P, is 0.567 day. At the times of maximum retardation the hydrogen lines appeared in emission near the phase of median increasing brightness. The dates of greatest retardation and greatest advance of the velocity-curve agree with those determined by Wairaven from the light-curve. Individual velocity-curves were derived for six groups of phase intervals, groups I and VI coinciding approximately with the dates of maximum retardation and groups III and IV coinciding approximately with the date of maximum advance. The velocity-curves in these six groups show only minor changes in shape. Most pronounced is a high, peaked maximum of velocity in groups I and VI, as shown by the hydrogen lines, Hy, Ho, and H~. Other minor differences in the velocity-curves corresponding to the six groups have been described and are considered as being probably real. The spectral types were determined for all plates by means Qf a sequence of standard stars. The re- suits show that the earliest type, A2, as estimated from the metallic lines and Ca ii 3933, occurs about 0.05 day earlier than minimum radial velocity. The latest spectral type, between A8 and Fl, occurs about 0.15 day earlier than maximum radial velocity. There is some indication that the spectral type had a larger range in groups I, II, and perhaps VI than in groups III, IV, and V