The low-mass white dwarf companion to β Crateris

We present FUSE H Lyman series spectroscopy of the hot white dwarf companion to the 4th magnitude A1 III star β Crt, which shows that it has an unusually low mass, M_WD = 0.43 M⊙, and has almost certainly evolved through binary interaction. This system could be a long-sought remnant of Algol-type evolution, although radial velocity measurements appear to show that the pair are not close. Instead, micro-variations in the proper motion of β Crt as measured by Hipparcos suggest that the period could be as high as ∼10 yr. However, a low-mass white dwarf in a system with a period ≳3 yr is difficult to explain by conventional models for binary evolution. We speculate on alternative models for the evolution of this system which involve an eccentric binary or multiple components.