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(Abridged) We present Keck, HST, and Gemini-North observations of the L4+L4binary HD 130948BC which together span ~70% of the binary's orbital period. Wedetermine a total dynamical mass of 0.109+/-0.002 Msun (114+/-2 Mjup). The fluxratio is near unity, so both components are unambiguously substellar for anyplausible mass ratio. An independent constraint on the age of the system isavailable from the G2V primary HD 130948A. The available indicators suggest anage comparable to the Hyades, with the most precise age being 0.79 Gyr based ongyrochronology. Therefore, HD 130948BC is now a unique benchmark among field Land T dwarfs, with a well-determined mass, luminosity, and age. We find thatsubstellar theoretical models disagree with our observations. Both componentsof HD 130948BC appear to be overluminous by a factor of ~2-3x compared toevolutionary models. The age of the system would have to be notably youngerthan the gyro age to ameliorate the luminosity disagreement. However,regardless of the adopted age, evolutionary and atmospheric models giveinconsistent results, indicating systematic errors in at least one class ofmodels, possibly both. The masses of HD 130948BC happen to be very near thetheoretical mass limit for lithium burning, and thus measuring the differentiallithium depletion between B and C will provide a uniquely discriminating testof theoretical models. The potential underestimate of luminosities byevolutionary models would have wide-ranging implications; therefore, a morerefined age estimate for HD 130948A is critically needed.

Dynamical Mass of the Substellar Benchmark Binary HD 130948BC