HD 49798/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 \pm 0.05 \mathrm { ~ { } M _ { \odot } } ) . 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 . HD 49798/RX J0648.0–4418 could be the progenitor of either a Type Ia supernova or of a non-recycled millisecond pulsars .