Next: EXTREME HELIUM STARS . EXTREME HELIUM STARS C.S. JEFFERY . ... Their exotic surface abundances point to previous evolution through the white dwarf sequence, followed by re-ignition due to a late helium shell flash or to a binary merger. ... I review the basic observational and theoretical properties of extreme helium star pulsations and show how these have been used to constrain evolutionary models, with particular reference to the cases of V652 Her and BX Cir. EXTREME HELIUM STARS . ...