A spectrqscopic binary at the M/L transition

We report the discovery of a single-lined spectroscopic binary with an ultracool dwarf (UCD) primary with a spectral type between M8 and L0.5. This system was discovered during the course of an ongoing survey to monitor L dwarfs for radial velocity variations and is the first known small-separation (a < 1 AU) spectroscopic binary among dwarfs at the M/L transition. Based on radial velocity measurements with a typical precision of 300 m s^-1, we estimate the orbital parameters of this system to be P = 246.73 ± 0.49 days, a₁ sin i = 0.159 ± 0.003 AU, and M₂ sin i = 0.2062(M₁ + M₂)^2/3 ± 0.0034 M⊙. Assuming a primary mass of M₁ = 0.08 M⊙ (based on spectral type), we estimate the secondary minimum mass to be M₂ sin i = 0.054 M⊙. With future photometric, spectroscopic, and interferometric observations it may be possible to determine the dynamical masses of both components directly, making this system one of the best-characterized UCD binaries known.