Resolved Spectroscopy of M Dwarf/l Dwarf Binaries. I. Denis J220002.05−303832.9ab

We present the discovery of the common proper motion M9 + L0 binary DENIS J220002.05−303832.9AB, identified serendipitously with the SpeX near infrared imager/spectrograph. Spectral types are derived from resolved near infrared spectroscopy of the wellseparated (1. 09±0. 06) components and comparison to equivalent data for M and L dwarf spectral standards. Physical association is deduced from the angular proximity of the sources, their common proper motion and their similar spectrophotometric distances (35±2 pc). The estimated distance of this pair implies a projected separation of 38±3 AU, wider than typical separations for other M dwarf/L dwarf binaries, but consistent with the maximum separation/total system mass trend previously identified by Burgasser et al. (2003). We discuss the DENIS 2200−3038AB system in context with other low mass binaries, and its role in studying dust formation processes and activity trends across the transition between the M and L dwarf spectral classes. Subject headings: binaries: visual — stars: individual (DENIS J220002.05−303832.9) — stars: low mass, brown dwarfs