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 1980 ’ s .
In order to explore this connection further , we have analysed the surface composition of six He-sdB stars , including LB 1766 , LB 3229 , SB 21 ( = Ton-S 137 = BPS 29503-0009 ) , BPS 22940–0009 , BPS 29496–0010 , 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 ] \approx - 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 .