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

A photometric and radial velocity survey of the six coolest extreme he lium (eHe) stars confirms that at least five, and probably all, of the se objects are both photometric and radial velocity variables. The sta rs have photometric variations on time-scales ranging from 8 to perhap s 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 app ear to represent a direct extension of the cool, hydrogen-deficient ca rbon star sequence to higher temperatures. Known eHe stars range in ef fective temperature (T(eff)) from 8400 to 55 000 K. The six stars incl uded in the survey have T(eff) of 8400 to 12400 K. The periods and tem peratures of these stars are consistent with evolutionary models for h ydrogen-deficient carbon stars evolved through the region of the HR di agram occupied by the cooler R Coronae Borealis (RCB) and hydrogen-def icient carbon (HdC) stars (T(eff) range of 5000 to 7500 K) to the eHe star region.