  We report the discovery of a single-lined spectroscopic binary with an Ultra
Cool 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 we estimate
the orbital parameters of this system to be P=246.73+/-0.49 d, a1
sin(i)=0.159+/-0.003 AU, M2 sin(i)=0.2062 (M1+M2)^(2/3)+/-0.0034 M_{\sun}.
Assuming a primary mass of M1=0.08M_{\sun} (based on spectral type), we
estimate the secondary minimum mass to be M2 sin(i)=0.054 M_{\sun}. 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.
