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White dwarfs represent the last stage of evolution of stars with mass lessthan about eight times that of the Sun and, like other stars, are often foundin binaries. If the orbital period of the binary is short enough, energy lossesfrom gravitational-wave radiation can shrink the orbit until the two whitedwarfs come into contact and merge. Depending on the component masses, themerger can lead to a supernova of type Ia or result in a massive white dwarf.In the latter case, the white dwarf remnant is expected to be highly magnetisedbecause of the strong magnetic dynamo that should arise during the merger, andbe rapidly spinning from the conservation of the orbital angular momentum. Herewe report observations of a white dwarf, ZTF J190132.9+145808.7, that exhibitsthese properties, but to an extreme: a rotation period of 6.94 minutes, amagnetic field ranging between 600 megagauss and 900 megagauss over itssurface, and a stellar radius of about 2,100 km, slightly larger than theradius of the Moon. Such a small radius implies that the star's mass is closeto the maximum white-dwarf mass, or Chandrasekhar mass. ZTF J190132.9+145808.7is likely to be cooling through the Urca processes (neutrino emission fromelectron capture on sodium) because of the high densities reached in its core.

A highly magnetised and rapidly rotating white dwarf as small as the Moon