SDSS 0423$-$04 to be double, the average spectral type of T0 being due to an L6 and T2 combination. Even more recently, the binary nature of Kelu-1, a 0$\farcs$29 pair, was revealed using Laser Guide Star adaptive optics on the Keck telescope (Liu & Leggett 2005; Gelino et al. 2006). Again, in neither case were these targets resolved in our NIRSPEC observations.

All of the observations were made using the NIRSPEC cryogenic spectrometer on the Keck II 10-m telescope on Mauna Kea, Hawaii. Detailed descriptions of the design and performance of this UCLA-built instrument are given elsewhere (McLean et al. 1998; 2000b). For this study, NIRSPEC was used in its cross-dispersed echelle mode. High resolution spectra are dispersed across the 1024$\times$1024 InSb detector at 0$\farcs$143 per pixel while the spatial scale in the cross-dispersed direction is 0$\farcs$19 per pixel. An independent slit-viewing camera with a scale of 0$\farcs$18 per pixel is available for centering and guiding. With the gratings used in NIRSPEC, the relationship between the blaze wavelength ($\lambda_{b}$) and echelle order number (m) is $m\lambda_{b}$ = 76.56 $\mu$m; together with the free spectral range (see below), this equation gives the order location of a given wavelength. The spectrometer was 