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Stellar-substellar binary systems are quite rare, and provide interestingbenchmarks. They constrain the complex physics of substellar atmospheres,because several physical parameters of the substellar secondary can be fixedfrom the much better characterized main sequence primary. We report thediscovery of CFBDS J111807-064016, a T2 brown dwarf companion to 2MASSJ111806.99-064007.8, a low-mass M4.5-M5 star. The brown-dwarf was identifiedfrom the Canada France Brown Dwarf Survey. At a distance of 50-120 pc, the 7.7arcsec angular separation corresponds to projected separations of 390-900 AU.The primary displays no Halpha emission, placing a lower limit on the age ofthe system of about 6 Gyr. The kinematics is also consistent with membership inthe old thin disc. We obtained near-infrared spectra, which together withrecent atmosphere models allow us determine the effective temperature andgravity 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, and0.06 to 0.07 Msol for the T dwarf. This system is a particularly valuablebenchmark because the brown dwarf is an early T: the cloud-clearing that occursat the L/T transition is very sensitive to gravity, metallicity, and detaileddust properties, and produces a large scatter in the colours. This T2 dwarf,with its metallicity measured from the primary and its mass and gravity muchbetter constrained than those of younger early-Ts, will anchor ourunderstanding of the colours of L/T transition brown dwarfs. It is also one ofthe most massive T dwarfs, just below the hydrogen-burning limit, and all thismakes it a prime probe a brown dwarf atmosphere and evolution models.

CFBDS J111807-064016: A new L/T transition brown dwarf in a binary system