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Summary: **TITLE**
ASP Conference Series, Vol. **VOLUME**, **PUBLICATION YEAR**
**EDITORS**
Helium white dwarf mergers as progeny for extreme
helium stars
C. Simon Je ery
Armagh Observatory, College Hill, Armagh BT61 9DG, N. Ireland
Hideyuki Saio
Astronomical Institute, School of Science, Tohoku University, Sendai
980-8578, Japan
Abstract. The origin of extreme helium stars poses a signi cant puz-
zle for stellar astrophysics. This paper summarizes the observational data
concerning extreme helium stars in general, and one, V652 Her, in partic-
ular. One proposal involves the merger of two white dwarfs in a binary.
Here, the rapid accretion of helium onto a helium white dwarf is used to
simulate the merger of two helium white dwarfs. Helium-shell ignition
occurs at the boundary between degenerate and non-degenerate helium
after suÆcient helium has been accreted, causing the star to become a
yellow giant. Thereafter, the helium-burning shell burns in to the core,
and the star contracts towards the helium main sequence where it will
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