  White dwarfs represent the last stage of evolution of stars with mass less
than about eight times that of the Sun and, like other stars, are often found
in binaries. If the orbital period of the binary is short enough, energy losses
from gravitational-wave radiation can shrink the orbit until the two white
dwarfs come into contact and merge. Depending on the component masses, the
merger 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 magnetised
because of the strong magnetic dynamo that should arise during the merger, and
be rapidly spinning from the conservation of the orbital angular momentum. Here
we report observations of a white dwarf, ZTF J190132.9+145808.7, that exhibits
these properties, but to an extreme: a rotation period of 6.94 minutes, a
magnetic field ranging between 600 megagauss and 900 megagauss over its
surface, and a stellar radius of about 2,100 km, slightly larger than the
radius of the Moon. Such a small radius implies that the star's mass is close
to the maximum white-dwarf mass, or Chandrasekhar mass. ZTF J190132.9+145808.7
is likely to be cooling through the Urca processes (neutrino emission from
electron capture on sodium) because of the high densities reached in its core.
