  White dwarf stars are the most common stellar fossils. When in binaries, they
make up the dominant form of compact object binary within the Galaxy and can
offer insight into different aspects of binary formation and evolution. One of
the most remarkable white dwarf binary systems identified to date is AR Scorpii
(henceforth AR Sco). AR Sco is composed of an M-dwarf star and a
rapidly-spinning white dwarf in a 3.56-hour orbit. It shows pulsed emission
with a period of 1.97 minutes over a broad range of wavelengths, which led to
it being known as a white dwarf pulsar. Both the pulse mechanism and the
evolutionary origin of AR Sco provide challenges to theoretical models. Here we
report the discovery of the first sibling of AR Sco, J191213.72-441045.1
(henceforth J1912-4410), which harbours a white dwarf in a 4.03-hour orbit with
an M-dwarf and exhibits pulsed emission with a period of 5.30 minutes. This
discovery establishes binary white dwarf pulsars as a class and provides
support for proposed formation models for white dwarf pulsars.
