An ultramassive, fast-spinning white dwarf in a peculiar binary system

White dwarfs typically have masses in a narrow range centered at about 0.6 solar mass ([M.sub.[dot encircle]). Only a few ultramassive white dwarfs (mass > 1.2 [M.sub.[dot encircle]) are known. Those in binary systems are of particular interest, because a small amount of accreted mass could drive them above the Chandrasekhar limit, beyond which they become gravitationally unstable. Using data from the x-ray multimirror mission (XMM)--Newton satellite, we show that the x-ray pulsator RX J0648.0-4418 is a white dwarf with mass > 1.2 [M.sub.[dot encircle]), based on dynamical measurements only. This ultramassive white dwarf in a post--common envelope binary with a hot subdwarf can reach the Chandrasekhar limit, and possibly explode as a type la supernova, when its helium-rich companion will transfer mass at an increased rate through Roche lobe overflow.