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 the results of long-term photometric monitoring, we investigate the pulsation properties of these objects, the link between pulsations and mass 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-active 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 lower 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-amplitude declines and infrared excesses of the RCB stars, we suggest that the pulsation amplitude dictates whether significant mass loss occurs in these objects. The radial velocity-to-light amplitude (RV/V) ratio appears to be temperature dependent, but we suspect that this is not an intrinsic effect. The 7000-K group of RCB stars have RV/V approximate to 50 km s(-1) mag(-1), which is similar to radially pulsating Cepheids. The similar pulsation properties of the RCB and HdC stars and the cooler eHe stars provide further evidence that these types of stars are closely related.