I thank W.Hartmann from SRON for providing the non-LTE white dwarf atmosphere spectra. I thank D. Bhattacharya for discussions. I thank Gene Magnier for making me the positions of the M31 Cepheids and blue stars available. I thank E.P.J. van den Heuvel for reading the manuscript. I thank an anonymous referee for critical reading of the manuscript and useful comments. P.K. is a Human Capital and Mobility fellow. This research was supported in part by the Netherlands Organisation for Scientific Research (NWO) through Spinoza Grant 08-0 to E.P.J. van den Heuvel.
© European Southern Observatory (ESO) 1999
Online publication: March 18, 1999