We have obtained intermediate resolution spectra of eleven candidate brown dwarf members of the Pleiades open cluster using the Keck II telescope and LRIS spectrograph. Our primary goal was to determine the location of the "lithium depletion edge" in the Pleiades and hence to derive a precise age for the cluster. All but one of our 11 program objects have radial velocities appropriate for Pleiades members, have moderately strong Hα emission, and have spectral types M6 to M8.5 as expected from their (R-I) c colors.We have constructed a color-magnitude diagram for the faint end of the Pleiades main sequence, including only stars for which high S/N spectra in the region of the lithium λ6708Å absorption line have been obtained. These data allow us to accurately determine the Pleiades single-star lithium depletion edge at I c0 = 17.80, (R-I) c0 = 2.20, spectral type = M6.5. By reference to theoretical evolutionary models, this converts fairly directly into an age for the Pleiades of τ = 125 Myr. This is significantly older than the age that is normally quoted, but does agree with some other recent estimates.

We present spatially separated optical spectra of the components of the young hierarchical quadruple GG Tau. Spectra of GG Tau Aa and Ab (separation 0. ′′ 25 ∼ 35 AU) were obtained with the Faint Object Spectrograph aboard the Hubble Space Telescope. Spectra of GG Tau Ba and Bb (separation 1. ′′ 48 ∼ 207 AU) were obtained with both the HIRES and the LRIS spectrographs on the W. M. Keck telescopes. The components of this mini-cluster, which span a wide range in spectral type (K7 -M7), are used to test both evolutionary models and the temperature scale for very young, low mass stars under the assumption of coeval formation. Of the evolutionary models tested, those of Baraffe et al. (1998) yield the most consistent ages when combined with a temperature scale intermediate between that of dwarfs and giants. The version of the Baraffe et al. models computed with a mixing length nearly twice the pressure scale height is of particular interest as it predicts masses for GG Tau Aa and Ab that are in agreement with their dynamical mass estimate.Using this evolutionary model and a coeval (at 1.5 Myrs) temperature scale, we find that the coldest component of the GG Tau system, GG Tau Bb, is substellar with a mass of 0.044 ± 0.006 M ⊙ . This brown dwarf companion is especially intriguing as it shows signatures of accretion, although this accretion is not likely to alter its mass significantly. GG Tau Bb is currently the lowest mass, spectroscopically confirmed companion to a T Tauri star, and is one of the coldest, lowest mass T Tauri objects in the Taurus-Auriga star forming region.

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