Final mixing in extreme horizontal branch stars.

When the hydrogen-rich envelope of an evolved horizontal branch structure is too small, with envelope mass not greater than 0.02 M(0), to allow the development of the asymptotic giant branch phase, a final helium-shell flash develops when the star is already on its white dwarf cooling sequence. The flash is strong enough to reach the hydrogen-rich layers. Substantial mixing and/or envelope ejection follow, together with some neutron nucleosynthesis.