HR2875: Spectroscopic discovery of the first B star + 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 HR2875. This is the first time that a non-interacting white dwarf$+$ B star binary has been discovered; previously, the the earliest type star known with a white dwarf companion was Sirius (A1V). Since the white dwarf must have evolved from a main sequence progenitor with a mass greater than that of a B5V star ($\geq$6.0M$_\odot$), this places a lower limit on the maximum mass for white dwarf progenitors, with important implications for our knowledge of the initial-final mass relation. Assuming a pure-hydrogen atmospheric composition, we constrain the temperature of the white dwarf to be between 39,000K and 49,000K. We also argue that this degenerate star is likely to have mass significantly greater than the mean mass for white dwarf stars ($\approx$0.55M$_\odot$). 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.