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(Abridged) We present multi-epoch imaging of the T5.0+T5.5 binary 2MASSJ1534-2952AB obtained with the Keck laser guide star adaptive optics system.Combined with an extensive (re-)analysis of archival HST imaging, we find atotal mass of 0.056+/-0.003 Msun (59+/-3 Mjup). This is the first field binaryfor which both components are directly confirmed to be substellar. This is alsothe coolest and lowest mass binary with a dynamical mass determination to date.Using evolutionary models, we derive an age of 0.78+/-0.09 Gyr for the system,and we find Teff = 1028+/-17 K and 978+/-17 K and masses of 0.0287+/-0.0016Msun (30.1+/-1.7 Mjup) and 0.0269+/-0.0016 Msun (28.2+/-1.7 Mjup) for theindividual components. These precise measurements generally agree with previousstudies of T dwarfs and affirm the current theoretical models. However, (1) thetemperatures are about 100 K cooler than derived for similar objects andsuggest that the ages of field brown dwarfs may be overestimated. Also, (2) theH-R diagram positions are discrepant with current predictions. While this mayarise from large errors in the evolutionary models, the likely cause is amodest (~100 K) overestimate in temperatures determined from model atmospheres.We discuss future tests of theory as the sample of substellar dynamical massesincreases. In particular, we suggest that low-mass field binaries withdynamical masses ("mass benchmarks") can serve as reference points for Teff andlog(g) and thereby constrain ultracool atmosphere models, as good as or evenbetter than single brown dwarfs with age estimates ("age benchmarks").

Keck Laser Guide Star Adaptive Optics Monitoring of 2MASS J1534-2952AB: First Dynamical Mass Determination of a Binary T Dwarf