HR 2875: spectroscopic discovery of the first B star plus white dwarf binary

We report the discovery,in an Extreme Ultraviolet Explorer (EUVE) short-wavelength spectrum, of an unresolved hot white dwarf companion to the 5th magnitude B5Vp star HR 2875. This is the first time that a non-interacting white dwarf + B star binary has been discovered: previously, the earliest type of star known with a white dwarf companion was Sirius (A1V). As the white dwarf must have evolved from a main-sequence-progenitor with a mass greater than that of a B5V star (greater than or equal to 6.0 M-circle dot), this places a lower limit on the maximum mass for white dwarf progenitors; with important implications for our knowledge of the initial-final mass relation. Assuming a pure-hydrogen atmospheric composition, we constrain the temperature of the white dwarf to be between 39 000 and 49 000 K. We also argue that this degenerate star is likely to have a mass significantly greater than the mean mass for white dwarf stars (approximate to 0.55 M-circle dot). Finally, we suggest that other bright B stars (e.g. theta Hya) detected in the extreme ultraviolet surveys of the ROSAT Wide Field Camera and EUVE may also be hiding hot white dwarf companions.