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We report the identification of the M9 dwarf SDSS J000649.16-085246.3 as aspectral binary and radial velocity variable with components straddling thehydrogen burning mass limit. Low-resolution near-infrared spectroscopy revealsspectral features indicative of a T dwarf companion, and spectral templatefitting yields component types of M8.5\pm0.5 and T5\pm1. High-resolutionnear-infrared spectroscopy with Keck/NIRSPEC reveals pronounced radial velocityvariations with a semi-amplitude of 8.2\pm0.4 km/s. From these we determine anorbital period of 147.6\pm1.5 days and eccentricity of 0.10\pm0.07, making SDSSJ0006-0852AB the third tightest very low mass binary known. This system is alsofound to have a common proper motion companion, the inactive M7 dwarf LP704-48, at a projected separation of 820\pm120 AU. The lack of H-alpha emissionin both M dwarf components indicates that this system is relatively old, asconfirmed by evolutionary model analysis of the tight binary. LP 704-48/SDSSJ0006-0852AB is the lowest-mass confirmed triple identified to date, and one ofonly seven candidate and confirmed triples with total masses below 0.3 Msuncurrently known. We show that current star and brown dwarf formation modelscannot produce triple systems like LP 704-48/SDSS J0006-0852AB, and we rule outKozai-Lidov perturbations and tidal circularization as a viable mechanism toshrink the inner orbit. The similarities between this system and the recentlyuncovered low-mass eclipsing triples NLTT 41135AB/41136 and LHS 6343ABC suggestthat substellar tertiaries may be common in wide M dwarf pairs.

Discovery of a Very Low Mass Triple with Late-M and T Dwarf Components: LP 704-48/SDSS J0006-0852AB