THE RADIAL-VELOCITY VARIATIONS OF COOL HYDROGEN-DEFICIENT CARBON STARS

We have obtained a series of radial velocities for many of the bright galactic R Coronae Borealis (RCB) and hydrogen-deficient carbon (HdC) stars, in order to characterize the pulsations of these types of cool hydrogen-deficient carbon stars. By comparing these velocities with th e results of long-term photometric monitoring, we investigate the puls ation properties of these objects, the link between pulsations and mas s loss, and the relationship between these stars and the higher-T-eff hydrogen-deficient stars, the extreme helium (eHe) stars. We find that most of the RCB stars have radial velocity and V light amplitudes of 10-20 km s(-1) and 0.2-0.3 mag, respectively. Pulsationally more-activ e stars such as RY Sgr are rare. Only one other RCB star, RT Nor, has a similar photometric amplitude to RY Sgr, but this star has a much lo wer radial velocity amplitude. With only one exception, HD 175893, all of the HdC stars have a lower pulsation amplitude than the RCB stars. Since the RCB and HdC stars appear to be distinguished by the large-a mplitude declines and infrared excesses of the RCB stars, we suggest t hat the pulsation amplitude dictates whether significant mass loss occ urs in these objects. The radial velocity-to-light amplitude (RV/V) ra tio appears to be temperature dependent, but we suspect that this is n ot an intrinsic effect. The 7000-K group of RCB stars have RV/V approx imate to 50 km s(-1) mag(-1), which is similar to radially pulsating C epheids. The similar pulsation properties of the RCB and HdC stars and the cooler eHe stars provide further evidence that these types of sta rs are closely related.