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We present Keck LGS AO imaging and aperture masking observations of the M8+L7binary LHS 2397aAB. Together with archival HST, Gemini-North, and VLT data, ourobservations span 11.8 years of the binary's 14.2-year orbital period. Wedetermine a total dynamical mass of 0.146+/-0.014 MSun (153+/-15 MJup). Usingthe combined observational constraints of the total mass and individualluminosities, the Tucson (Lyon) evolutionary models give an age for the systemof 1.6 Gyr (1.8 Gyr), consistent with its space motion. We also use thesemodels to determine the mass ratio, giving individual masses of 0.0839+/-0.0011MSun (0.0848+/-0.0011 MSun) for LHS 2397aA and 0.061+/-0.013 MSun(0.060+/-0.010 MSun) for LHS 2397aB. Because LHS 2397aB is very close to thetheoretical mass-limit of lithium burning, measuring its lithium depletionwould uniquely test substellar models. We estimate a spectral type of L7+/-1for LHS 2397aB, making it the first L/T transition object with a dynamical massdetermination. This enables a precise estimate of its effective temperaturefrom Tucson (Lyon) models of 1460+/-40 K (1440+/-40 K), which is 200 K higherthan estimates for young late-L companions but consistent with older late-Lfield dwarfs, supporting the idea that the temperature of the L/T transition issurface gravity dependent. Comparing our temperature estimate for LHS 2397aB tothose derived from spectral synthesis modeling for similar objects revealsconsistency between evolutionary and atmospheric models at the L/T transition,despite the currently limited understanding of this phase of substellarevolution. Future dynamical masses for L/T binaries spanning a range of surfacegravity and age will provide the next critical tests of substellar models atthe L/T transition.

Keck Laser Guide Star Adaptive Optics Monitoring of the M8+L7 Binary LHS 2397aAB: First Dynamical Mass Benchmark at the L/T Transition