LUMINOSITY AND RADIAL-VELOCITY VARIATIONS OF THE 6 COOLEST EXTREME HELIUM STARS

A photometric and radial velocity survey of the six coolest extreme helium (eHe) stars confirms that at least five, and probably all, of these objects are both photometric and radial velocity variables. The stars have photometric variations on time-scales ranging from 8 to perhaps 25 d, and radial velocity amplitudes of 6 to 20 km s-1. In at least one case the variations appear to indicate that the star is pulsating in a radial mode. The pulsational characteristics of these objects appear to represent a direct extension of the cool, hydrogen-deficient carbon star sequence to higher temperatures. Known eHe stars range in effective temperature (T(eff)) from 8400 to 55 000 K. The six stars included in the survey have T(eff) of 8400 to 12400 K. The periods and temperatures of these stars are consistent with evolutionary models for hydrogen-deficient carbon stars evolved through the region of the HR diagram occupied by the cooler R Coronae Borealis (RCB) and hydrogen-deficient carbon (HdC) stars (T(eff) range of 5000 to 7500 K) to the eHe star region.