  We discuss the behaviour of the JHKL'M' colors of L and T dwarfs, based on
photometric and spectroscopic data obtained at the United Kingdom Infrared
Telescope in Hawaii. We have measured the first accurate M' photometry for L
and T dwarfs. The K-M' colors of T dwarfs are much bluer than predicted by
published models, suggesting that CO may be more abundant than expected, as has
been found spectroscopically for the T6 dwarf Gl 229B. We also find that K-L'
increases monotonically through most of the M, L, and T subclasses, but it is
approximately constant between types L6 and T5, due to the onset of methane
absorption at the blue edge of the L' bandpass. The JHK colors of L dwarfs show
significant scatter, suggesting variations in the amount and properties of
photospheric dust, and indicating that it may not be possible to associate a
unique effective temperature with a given L spectral type. The H-K colors of
the later T dwarfs also show some scatter which we suggest is due to variations
in pressure-induced molecular hydrogen opacity, which is sensitive to gravity
(or age for a brown dwarf) and metallicity.

Astrophysics