2010MNRAS.409..582N

The connection between helium-rich hot subdwarfs of spectral types O and B (He-sdB) has been relatively unexplored since the latter were found in significant numbers in the 1980s. In order to explore this connection further, we have analysed the surface composition of six He-sdB stars, including LB1766, LB3229, SB21 (= Ton-S137 = BPS29503-0009), BPS22940-0009, BPS29496-0010 and BPS 22956-0094. Opacity-sampled line-blanketed model atmospheres have been used to derive atmospheric properties and elemental abundances. All the stars are moderately metal poor compared with the Sun ([ Fe/H ] ~ -0.5). Four stars are nitrogen rich, two of these are carbon rich and at least four appear to be neon rich. The data are insufficient to rule out binarity in any of the sample. The surface composition and locus of the N-rich He-sdBs are currently best explained by the merger of two helium white dwarfs, or possibly by the merger of a helium white dwarf with a post-sdB white dwarf. C-rich He-sdBs require further investigation.