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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 theKeck II Telescope using laser guide star adaptive optics. Physical associationis deduced from the angular proximity of the components and constraints ontheir common proper motion. We have obtained a near-infrared spectrum of thebinary and find that it is best described by an L5+/-0.5 primary and anL7.5+/-0.5 secondary. Model-dependent masses predict that the two componentsstraddle the hydrogen burning limit threshold with the primary likely stellarand the secondary likely substellar. The properties of this sytem - closeprojected separation (1.8+/-0.3 AU) and near unity mass ratio - are consistentwith previous results for very low-mass field binaries. The relatively shortestimated orbital period of this system (~7-12 yr) makes it a good target fordynamical mass measurements. Interestingly, the system's angular separation isthe tightest yet for any very low-mass binary published from a ground-basedtelescope and is the tightest binary discovered with laser guide star adaptiveoptics to date.Comment: 10 pages, 3 figures; accepted for publication to A

Discovery of a 66 mas Ultracool Binary with Laser Guide Star Adaptive Optics