5 Conclusion

We have obtained intermediate dispersion spectra of the supposed hot subdwarf binary BI Lyn. These have been analysed, together with the overall flux distribution, to establish the dimensions of both hot and cool components. The hot component is clearly demonstrated to be a low-mass hydrogen-deficient star with low surface gravity. The cool component is most likely a giant of approximately one to a few solar masses. The hydrogen-deficiency of the hot star is probably the result of a common-envelope phase during which the outer envelope was entirely removed or transferred to the cool companion. The hot star luminosity suggests that it lies on a post-AGB evolution track. Previous reports of variability in light and H$\alpha$ emission in BI Lyn are consistent with the behaviour of other H-deficient giants, being caused by pulsations and mass transfer respectively. Further observations are required to determine the orbital period and mass ratio, to verify the pulsation hypothesis and to correlate the H$\alpha$ behaviour with orbital phase.

Note added in proof: A striking similarity between BI Lyn and HD 128220 has been drawn to our attention. The latter is a binary containing an O subdwarf and a rapidly rotating G-type primary, probably spun up by mass exchange (Howard I. D., & Heber U. 1990, PASP, 102, 912). Thus both systems consist of a hot post-AGB star and a more-massive rapidly-rotating cool companion.

Acknowledgements

This research has made extensive use of software provided through UK PPARC Collaborative Computational Project No. 7 for "the Analysis of Astronomical Spectra'', software and facilities provided through the UK PPARC Starlink project and the SIMBAD database, operated at CDS, Strasbourg, France. We acknowledge financial support from the Northern Ireland Department of Culture, Arts and Leisure.