  RX J0648.0-4418 is a post common-envelope X-ray binary composed of a hot
subdwarf and one of the most massive white dwarfs with a dynamical mass
measurement (1.28+/-0.05 M_sun). This white dwarf, with a spin period of 13.2
s, rotates more than twice faster than the white dwarf in the cataclysmic
variable AE Aqr. The current properties of these two binaries, as well as their
future evolution, are quite different, despite both contain a fast-spinning
white dwarf. RX J0648.0-4418 could be the progenitor of either a Type Ia
supernova or of a non-recycled millisecond pulsars.
