  We report the discovery of a transiting brown dwarf companion to KOI-205, a
K0 main-sequence star, in a 11.720125-day period orbit. The transits were
detected by the Kepler space telescope, and the reflex motion of the star was
measured using radial velocity observations obtained with the SOPHIE
spectrograph. The atmospheric parameters of the host stars were determined from
the analysis of high-resolution, high signal-to-noise ratio ESPaDOns spectra
obtained for this purpose. Together with spectrophotometric measurements
recovered from the literature, these spectra indicate that the star is a mildly
metallic K0 dwarf with Teff 5237 $\pm$ 60 K. The mass of the companion is 39.9
$\pm$ 1.0 MJup and its radius is 0.81 $\pm$ 0.02 RJup, in agreement with
current theoretical predictions. This is the first time a bona fide brown dwarf
companion is detected in orbit around a star of this type. The formation and
orbital evolution of brown dwarf companions is briefly discussed in the light
of this new discovery.
