  We present the discovery of 2MASS J21321145+1341584AB as a closely separated
(0.066") very low-mass field dwarf binary resolved in the near-infrared by the
Keck II Telescope using laser guide star adaptive optics. Physical association
is deduced from the angular proximity of the components and constraints on
their common proper motion. We have obtained a near-infrared spectrum of the
binary and find that it is best described by an L5+/-0.5 primary and an
L7.5+/-0.5 secondary. Model-dependent masses predict that the two components
straddle the hydrogen burning limit threshold with the primary likely stellar
and the secondary likely substellar. The properties of this sytem - close
projected separation (1.8+/-0.3 AU) and near unity mass ratio - are consistent
with previous results for very low-mass field binaries. The relatively short
estimated orbital period of this system (~7-12 yr) makes it a good target for
dynamical mass measurements. Interestingly, the system's angular separation is
the tightest yet for any very low-mass binary published from a ground-based
telescope and is the tightest binary discovered with laser guide star adaptive
optics to date.
