7 Conclusions

We have analysed representative samples of sdB stars having apparently single or composite spectra. The atmospheric properties of the sdB stars were measured by comparing moderate-resolution blue spectra with theoretical models. In the case of sdBs with composite spectra, the atmospheric properties of the cool companions were measured from the blue spectra and from near-infrared spectra, where the infrared calcium triplet provides an invaluable surface gravity indicator.

Both samples covered a similarly wide range in T $_{\rm eff}$ and log g. However the composite sdB stars invariably have lower helium abundances in their atmospheres than the single-spectrum sdBs. Although we cannot entirely explain this phenomenon, we suggest that it may be due to tidal effects disrupting diffusive separation in short-period systems more than in long-period systems. Some of the composite sdB stars also showed depletions of metals including silicon, carbon and magnesium.

Assuming a typical surface luminosity representing all subdwarf B stars of log $(L_{\rm sdB}/\mbox{\,$L_{\odot}$ }) = 1.40 \pm 0.13$, the majority of the companions of composite sdBs have luminosities in the range $0.4 \le L_{\rm cool}/\mbox{\,$L_{\odot}$ }\le 2.6$, consistent with being main-sequence stars of about $1\pm0.2 \mbox{\,$M_{\odot}$ }$. This supports the hypothesis that composite sdB stars are the result of Roche lobe overflow near the red-giant tip in a low-mass binary with nearly equal initial masses (Green et al. 2001).

This is the first time that an attempt has been made to model accurately the spectra of composite subdwarf B stars and to measure the cool star luminosity using the infrared calcium triplet. Although successfully executed, higher resolution spectra will be needed to measure the metallicity of the cool star and hence the radius ratios with greater accuracy.

Acknowledgements

This research is supported by a grant to the Armagh Observatory from the Northern Ireland Department of Culture, Arts and Leisure and by the UK Particle Physics and Astronomy Research Council through the award of telescope time and travel grants. We are particularly grateful to Prof Philip Dufton and Drs Sabine Moehler and Betsy Green for helpful discussions and to Dr Don Pollacco for obtaining some of the observations.