type is also investigated. The effects of rotation, surface gravity and metallicity are explored in §4. A summary of the overall results and concluding remarks is given in §5.

Observations and Data Reduction
===============================

Targets and Instrumentation
---------------------------

Targets for the initial survey, the BDSS (M03), were selected primarily from well-known M dwarfs and from L and T dwarfs identified in the Two Micron All Sky Survey (2MASS; Kirkpatrick et al. 1997, 1999, 2000, 2001; Burgasser et al. 2000, 2002; Reid et al. 2000; Wilson et al. 2001), augmented with discoveries from the Deep Near-Infrared Survey of the Southern Sky (DENIS; Delfosse et al. 1997, 1999), the Sloan Digital Sky Survey (SDSS; Leggett et al. 2000; Geballe et al. 2002), and other investigations (Becklin & Zuckerman 1988; Ruiz, Leggett and Allard 1997). To ensure high signal-to-noise spectra for the high-resolution part of the survey, a subset of 12 of the brightest objects ($J=7-15$), spanning the spectral type range from M6 to T6, was selected. Of these, only 2MASS 0140+27 was not part of the initial survey. The M2.5 star G196-3A was also observed along with its L2 companion G196-3B, both examples of objects substantially younger than 1 Gyr (Rebolo et 