  The variable star AR Sco was recently discovered to pulse in brightness every
1.97 min from ultraviolet wavelengths into the radio regime. The system is
composed of a cool, low-mass star in a tight, 3.55 hr orbit with a more massive
white dwarf. Here we report new optical observations of AR Sco that show strong
linear polarization (up to 40%) which varies strongly and periodically on both
the spin period of the white dwarf and the beat period between the spin and
orbital period, as well as low level (< a few %) circular polarization. These
observations support the notion that, similar to neutron star pulsars, the
pulsed luminosity of AR Sco is powered by the spin-down of the rapidly-rotating
white dwarf which is highly magnetised (up to 500 MG). The morphology of the
modulated linear polarization is similar to that seen in the Crab pulsar,
albeit with a more complex waveform owing to the presence of two periodic
signals of similar frequency. Magnetic interactions between the two component
stars, coupled with synchrotron radiation from the white dwarf, power the
observed polarized and non-polarized emission. AR Scorpii is therefore the
first example of a white dwarf pulsar.
