Detached white-dwarf close-binary stars - CV's extended family

I review detached binaries consisting of white dwarfs with either other whi te dwarfs or low mass main-sequence stars in tight orbits around them; Orbi tal periods have been measured for 15 white dwarf/white dwarf systems and 2 2 white dwarf/M dwarf systems. While small compared to the number of period s known for CVs (>300), I argue that each variety of detached system has a space density an order of magnitude higher that of CVs. While theory matche s the observed distribution of orbital periods of the white dwarf/white dwa rf binaries, it predicts white dwarfs: of much lower mass than observed. Am ongst both types of binary are clear examples of helium core white dwarfs, as opposed to the usual CO composition; similar systems must exist amongst the CVs. White dwarf/M dwarf binaries suffer from selection effects which d iminish the numbers seen at long and short periods. They are useful for the study of irradiation; I discuss evidence to suggest that Balmer emission i s broadened by optical depth effects to an extent which limits its usefulne ss for imaging the secondary stars in CVs. (C) 2000 Elsevier Science B.V. A ll rights reserved.