  Stellar-substellar binary systems are quite rare, and provide interesting
benchmarks. They constrain the complex physics of substellar atmospheres,
because several physical parameters of the substellar secondary can be fixed
from the much better characterized main sequence primary. We report the
discovery of CFBDS J111807-064016, a T2 brown dwarf companion to 2MASS
J111806.99-064007.8, a low-mass M4.5-M5 star. The brown-dwarf was identified
from the Canada France Brown Dwarf Survey. At a distance of 50-120 pc, the 7.7
arcsec angular separation corresponds to projected separations of 390-900 AU.
The primary displays no Halpha emission, placing a lower limit on the age of
the system of about 6 Gyr. The kinematics is also consistent with membership in
the old thin disc. We obtained near-infrared spectra, which together with
recent atmosphere models allow us determine the effective temperature and
gravity of both components. From these parameters and the age constraint,
evolutionary models estimate masses of 0.10 to 0.15 Msol for the M dwarf, and
0.06 to 0.07 Msol for the T dwarf. This system is a particularly valuable
benchmark because the brown dwarf is an early T: the cloud-clearing that occurs
at the L/T transition is very sensitive to gravity, metallicity, and detailed
dust properties, and produces a large scatter in the colours. This T2 dwarf,
with its metallicity measured from the primary and its mass and gravity much
better constrained than those of younger early-Ts, will anchor our
understanding of the colours of L/T transition brown dwarfs. It is also one of
the most massive T dwarfs, just below the hydrogen-burning limit, and all this
makes it a prime probe a brown dwarf atmosphere and evolution models.
