  The transition between the L dwarf and T dwarf spectral classes is one of the
most remarkable along the stellar/brown dwarf Main Sequence, separating sources
with photospheres containing mineral condensate clouds from those containing
methane and ammonia gases. Unusual characteristics of this transition include a
1 micron brightening between late L and early T dwarfs observed in both
parallax samples and coeval binaries; a spike in the multiplicity fraction;
evidence of increased photometric variability, possibly arising from patchy
cloud structures; and a delayed transition for young, planetary-mass objects.
All of these features can be explained if this transition is governed by the
"rapid" (nonequlibrium) rainout of clouds from the photosphere, triggered by
temperature, surface gravity, metallicity and (perhaps) rotational effects.
While the underlying mechanism of this rainout remains under debate, the
transition is now being exploited to discover and precisely characterize tight
(< 1 AU) very low-mass binaries that can be used to test brown dwarf
evolutionary and atmospheric theories, and resolved binaries that further
constrain the properties of this remarkable transition.
