Luminosity and radial velocity variations of the six coolest extreme helium stars

Abstract

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