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We have been using Keck laser guide star adaptive optics to monitor theorbits of ultracool binaries, providing dynamical masses at lower luminositiesand temperatures than previously available and enabling strong tests oftheoretical models. (1) We find that model color-magnitude diagrams cannotreliably be used to infer masses as they do not accurately reproduce the colorsof ultracool dwarfs of known mass. (2) Effective temperatures inferred fromevolutionary model radii can be inconsistent with temperatures derived fromfitting observed spectra with atmospheric models by at most 100-300 K. (3) Forthe single pair of field brown dwarfs with a precise mass (3%) and agedetermination (~25%), the measured luminosities are ~2-3x higher than predictedby model cooling rates (masses inferred from Lbol and age are 20-30% largerthan measured). Finally, as the sample of binaries with measured orbits grows,novel tests of brown dwarf formation theories are made possible (e.g., testingtheoretical eccentricity distributions).

Testing Models with Brown Dwarf Binaries