Discovery of a tight brown dwarf companion to the low-mass star LHS 2397a

Using the adaptive optics system, Hokupa'a, at Gemini North, we have directly imaged a companion around the UKIRT faint standard M8 star LHS 2397a (FS 129) at a separation of 2.96 AU. Near-infrared photometry of the companion has shown it to be an L7.5 brown dwarf and confirmed the spectral type of the primary to be M8. We also derive a substellar mass of the companion of 0.068 M-circle dot, although masses in the range 0.061-0.069 M-circle dot are possible, and the primary mass is 0.090 M-circle dot (0.089-0.094 M-circle dot). Reanalysis of archival imaging from the Hubble Space Telescope has confirmed the secondary as a common proper motion object. This binary represents the first clear example of a brown dwarf companion within 4 AU of a low-mass star and should be one of the first late-L dwarfs to have a dynamical mass. As part of a larger survey of M8-L0 stars, this object may indicate that there is no brown dwarf desert around low-mass primaries.