  Brown dwarfs display Jupiter-like auroral phenomena such as magnetospheric
H$\alpha$ emission and coherent radio emission. Coherent radio emission is a
probe of magnetospheric acceleration mechanisms and provides a direct
measurement of the magnetic field strength at the emitter's location, both of
which are difficult to access by other means. Observations of the coldest brown
dwarfs (spectral types T and Y) are particularly interesting as their
magnetospheric phenomena may be very similar to those in gas-giant exoplanets.
Here we present 144 MHz radio and infrared adaptive optics observations of the
brown dwarf WISEP J101905.63+652954.2 made using the LOFAR and Keck telescopes
respectively. The radio data shows pulsed highly circularly polarised emission
which yields a rotation rate of $0.32\pm0.03$ hr$^{-1}$. The infrared imaging
reveals the source to be a binary with a projected separation of $423.0\pm1.6$
mas between components of spectral type T5.$5\pm0.5$ and T7.$0\pm0.5$. With a
simple "toy model" we show that the radio emission can in principle be powered
by the interaction between the two dwarfs with a mass-loss rates of at least
$25$ times the Jovian value. WISEP J101905.63+652954.2 is interesting because
it is the first pulsed methane dwarf detected in a low radio-frequency search.
Unlike previous gigahertz-frequency searches that were only sensitive to
objects with kiloGauss fields, our low-frequency search is sensitive to surface
magnetic fields of $\approx 50$ Gauss and above which might reveal the coldest
radio-loud objects down to planetary mass-scales.

Solar and Stellar Astrophysics;Earth and Planetary Astrophysics