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We present JHK photometry and spectroscopy for a sample of newly identified Land T dwarfs from the Sloan Digital Sky Survey (SDSS) and known dwarfs from theSDSS and the Two Micron All Sky Survey (2MASS). Spectral classification hasbeen carried out using four infrared indices; we identify 9 new L8-9.5 dwarfsand 14 new T dwarfs from SDSS, including the latest SDSS dwarf yet found, theT7 SDSS J1758+4633. We classify 2MASS J0415-0935 as T9, the latest and coolestdwarf found to date. We combine these results with previously published data toproduce a sample of 59 L dwarfs and 42 T dwarfs. We compare the near-IR colorsand absolute magnitudes of dwarfs near the L-T transition with predictions madeby models of the distribution and evolution of photospheric condensates. Thereis scatter in the spectral indices for L dwarfs, suggesting that these indicesare probing different levels of the atmosphere. The near-IR colors of the Ldwarfs also scatter, likely due to variations in the altitudes, spatialdistributions and thicknesses of the condensate clouds. We identify a group oflate L dwarfs that are blue for their spectral type and that have enhanced FeH,H2O and KI absorption, possibly due to an unusually small amount ofcondensates. The scatter seen in the H-K color for late T dwarfs can bereproduced by models with a range in surface gravity. The variation is probablydue to the effect on the K-band flux of pressure-induced H2 opacity. Thecorrelation of H-K color with gravity is supported by the strengths of theJ-band KI doublet. Gravity is closely related to mass for field T dwarfs andthe gravities implied by the H-K colors indicate that the T dwarfs in oursample have masses in the range 15-75 M_Jupiter, except for SDSS J1110+0116which possibly has a mass of 10-15 M_Jupiter.Comment: 59 pages includes 12 Tables and 9 Figures, Figure 1a&b as jpgs under  postscript format option for astroph. Accepted for publication in AJ to  appear June 2004. Version 2 has a coordinate error corrected in Table 9.  Version 3 has corrected indices and spectral type (T5.5) for SDSS111

Near-Infrared Photometry and Spectroscopy of L and T Dwarfs: The Effects of Temperature, Clouds, and Gravity