Atmospheric analyses of helium-rich subdwarf B stars: 
are they products of He+He white dwarf mergers ? 

A. Ahmad, C.S. Jeffery, V.M. Woolf

Armagh Observatory, College Hill, Armagh BT61 9DG

Most subdwarf B stars have spectra that show helium abundances
depleted to a lesser or greater extent. This is attributed to
diffusion and gravitational settling.  However a significant number of
stars with similar temperatures and gravities show relatively strong
helium spectra. The question is how these helium-rich stars are
related to their helium-deficient counterparts.  It has been proposed
(Aznar Cuadrado \& Jeffery 2002) that the He-rich sdB stars represent
the product of He+He white dwarf mergers, as originally suggested by
Iben (1990) and more recently modelled by Saio & Jeffery (2000). If
so, it is expected that the He-rich sdBs should show other
composition anomalies, particularly in carbon, nitrogen and oxygen.
They should be quite distinct from the He-poor sdBs produced by
close-binary evolution. However there have been few systematic studies
of the atmospheres of He-rich sdB stars to date. We shall present
preliminary analyses of a number of He-rich sdB stars in order to
address this question.