  Accreting white dwarfs are often found in close binary systems with orbital
periods ranging from tens of minutes to several hours. In most cases, the
accretion process is relatively steady, with significant modulations only
occurring on time-scales of ~days or longer. Here, we report the discovery of
abrupt drops in the optical luminosity of the accreting white dwarf binary
system TW Pictoris by factors up to 3.5 on time-scales as short as 30 minutes.
The optical light curve of this binary system obtained by the Transiting
Exoplanet Survey Satellite (TESS) clearly displays fast switches between two
distinct intensity modes that likely track the changing mass accretion rate
onto the white dwarf. In the low mode, the system also displays
magnetically-gated accretion bursts, which implies that a weak magnetic field
of the white dwarf truncates the inner disk at the co-rotation radius in this
mode. The properties of the mode switching observed in TW Pictoris appear
analogous to those observed in transitional millisecond pulsars, where similar
transitions occur, although on timescales of ~tens of seconds. Our discovery
establishes a previously unrecognised phenomenon in accreting white dwarfs and
suggests a tight link to the physics governing magnetic accretion onto neutron
stars.
