A New Formation Mechanism for the Hottest Horizontal-Branch Stars
  Stars with very large mass loss on the red-giant branch can undergo the helium flash while descending the white-dwarf cooling curve. Under these conditions the flash convection zone will mix the hydrogen envelope with the hot helium- burning core. Such ``flash-mixed'' stars will arrive on the extreme horizontal branch (EHB) with helium- and carbon-rich envelopes and will lie at higher temperatures than the hottest canonical (i.e., unmixed) EHB stars. Flash mixing provides a new evolutionary channel for populating the hot end of the EHB and may explain the origin of the high gravity, helium-rich sdO and sdB stars. 
Allen V. Sweigart (NASA/GSFC), Thomas M. Brown (STScI), Thierry Lanz,   Wayne B. Landsman, & Ivan Hubeny (NASA/GSFC)