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