  A long standing and unverified prediction of binary star evolution theory is
the existence of a population of white dwarfs accreting from sub-stellar donor
stars. Such systems ought to be common, but the difficulty of finding them,
combined with the challenge of detecting the donor against the light from
accretion means that no donor star to date has a measured mass below the
hydrogen burning limit. Here we apply a technique which allows us to reliably
measure the mass of the unseen donor star in eclipsing systems. We are able to
identify a brown dwarf donor star, with a mass of 0.052+/-0.002 Msun. The
relatively high mass of the donor star for its orbital period suggests that
current evolutionary models may underestimate the radii of brown dwarfs.
