Extreme Helium Stars

Abstract

Results of research into the nature of extreme helium stars are reviewed, covering published and unpublished work carried out over the past 20 years. Three approaches are being used to investigate the nature and evolution of extreme helium stars. Abundance analysis from model atmospheres provides information useful in interpreting the evolution of stars. Studies of amplitude variations in the optical light emitted from extreme helium stars often can yield information about the internal structures of the stars, and also are important because of the suggested evolutionary connection of extreme helium stars with R CrB stars. Mass-loss rate determinations are important because the presence of mass-loss is indicative of more dramatic mass-loss that may have resulted in the loss of the original hydrogen envelope.