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We have identified CFBDS J1458+10 as a 0.11" (2.6 AU) physical binary usingKeck laser guide star adaptive optics imaging and have measured a distance of23.1+/-2.4 pc to the system based on near-IR parallax data from CFHT. Theintegrated-light near-IR spectrum indicates a spectral type of T9.5, and modelatmospheres suggest a slightly higher temperature and surface gravity than theT10 dwarf UGPS J0722-05. Thus, CFBDS J1458+10AB is the coolest brown dwarfbinary found to date. Its secondary component has an absolute H-band magnitudethat is 1.9+/-0.3 mag fainter than UGPS J0722-05, giving an inferred spectraltype of >T10. The secondary's bolometric luminosity of ~2 x 10^{-7} L_sun makesit the least luminous known brown dwarf by a factor of 4-5. By comparing toevolutionary models and T9-T10 objects, we estimate a temperature of 370+/-40 Kand a mass of 6-15 Mjup for CFBDS J1458+10B. At such extremes, atmosphericmodels predict the onset of novel photospheric processes, namely the appearanceof water clouds and the removal of strong alkali lines, but their impact on theemergent spectrum is highly uncertain. Our photometry shows that strong CH4absorption persists at H-band; the J-K color is bluer than the latest known Tdwarfs but not as blue as predicted by current models; and the J-H colordelineates a possible inflection in the blueward trend for the latest T dwarfs.Given its low luminosity, atypical colors and cold temperature, CFBDS J1458+10Bis a promising candidate for the hypothesized Y spectral class. However,regardless of its ultimate classification, CFBDS J1458+10AB provides a newbenchmark for measuring the properties of brown dwarfs and gas-giant planets,testing substellar models, and constraining the low-mass limit for starformation.

CFBDSIR J1458+1013B: A Very Cold (>T10) Brown Dwarf in a Binary System