  We present near-infrared spectroscopy for 52 ultracool dwarfs, including two
newly discovered late-M dwarfs, one new late-M subdwarf candidate, three new L
and four new T dwarfs. We also present parallaxes and proper motions for 21 of
them. Four of the targets presented here have previous parallax measurements,
while all the others are new values. This allow us to populate further the
spectral sequence at early types (L0-L4). Combining the astrometric parameters
with the new near-infrared spectroscopy presented here, we are able to
investigate further the nature of some of the objects. In particular, we find
that the peculiar blue L1 dwarf SDSS J133148.92-011651.4 is a metal-poor
object, likely a member of the galactic thick disk. We discover a new M
subdwarf candidate, 2MASS J20115649-6201127. We confirm the low-gravity nature
of EROS-MP J0032-4405, DENIS-P J035726.9-441730, and 2MASS J22134491-2136079.
We present two new metal-poor dwarfs: the L4pec 2MASS J19285196-4356256 and the
M7pec SIPS2346-5928. We also determine the effective temperature and bolometric
luminosity of the 21 targets with astrometric measurements, and we obtain a new
polynomial relation between effective temperature and near-infrared spectral
type. The new fit suggests a flattening of the sequence at the transition
between M and L spectral types. This could be an effect of dust formation, that
causes a more rapid evolution of the spectral features as a function of the
effective temperature.
