HIPPARCOS OBSERVATIONS OF HYDROGEN-DEFICIENT CARBON STARS

Parallax measurements for 21 hydrogen-deficient carbon stars have been made by the Hipparcos satellite. These stars include most of the brig hter R Coronae Borealis (RCB) variables, other cool hydrogen-deficient carbon (Hdc) stars, and several higher-T-eff extreme helium (eHe) sta rs. Most of these stars have either negative or statistically insignif icant parallaxes, indicating that they lie beyond the detection capabi lity of Hipparcos. Although the distances to the galactic hydrogen-def icient carbon stars remain unknown, at least the Hipparcos observation s do confirm that these objects must have high luminosity like the LMC RCB stars, for which M-bol = -4 to -5. Based upon Hipparcos proper mo tions, we derive UVW velocities for the RCB and HdC stars, assuming M- bol = -3 and -5. The UW-velocity dispersion of the RCB/HdC stars is si milar to that already reported for the eHe stars, further supporting t he idea that these groups of stars have predominantly bulge distributi ons. However, UW Cen may be a second example of a halo RCB star curren tly seen transitting the Galactic plane.