  We present a systematic study of the physical properties of late-M dwarfs
based on high-quality dynamical mass measurements and near-infrared (NIR)
spectra. We use astrometry from Keck NGS and LGS AO imaging to determine orbits
for late-M binaries. We find that LP 349-25 (M7.5+M8) is a pair of young brown
dwarfs (Mtot = 0.120 Msun) for which Lyon and Tucson evolutionary models
jointly predict an age of 140+/-30 Myr. This is consistent with the age of the
Pleiades, but at least LP 349-25A defies the empirical Pleiades lithium
depletion boundary, implying that the system is in fact older and that
evolutionary models underpredict the component luminosities. We find that LHS
1901AB (M6.5+M6.5) is a pair of very low-mass stars (Mtot = 0.194 Msun) with
model-derived ages consistent with limits from its lack of activity (> 6 Gyr).
Our improved orbit for Gl 569Bab (M8.5+M9) results in a higher mass for this
binary (Mtot = 0.140 Msun) compared to previous work (0.125 Msun). We use these
masses along with our published results for 2MASS J2206-2047AB (M8+M8) to test
four sets of ultracool model atmospheres currently in use. Fitting these models
to our NIR integrated-light spectra provides temperature estimates warmer by
~250 K than those derived independently from Dusty evolutionary models given
the measured masses and luminosities. We propose that model atmospheres are
more likely to be the source of this discrepancy, as it would be difficult to
explain a uniform temperature offset over such a wide range of masses, ages,
and activity levels in the context of evolutionary models. Our results contrast
those of Konopacky et al. as we find an opposite and smaller mass discrepancy
from what they report when we adopt their model-testing approach since our Teff
estimates from fitting spectra are ~650 K higher than from their fitting of
broadband photometry alone.
