  We present the results of a Hubble Space Telescope NICMOS imaging survey of
22 T-type field brown dwarfs. Five are resolved as binary systems with angular
separations of 0"05-0"35, and companionship is established on the basis of
component F110W-F170M colors (indicative of CH4 absorption) and low
probabilities of background contamination. Prior ground-based observations show
2MASS 1553+1532AB to be a common proper motion binary. The properties of these
systems - low multiplicity fraction (11[+7][-3]% resolved, as corrected for
sample selection baises), close projected separations (a = 1.8-5.0 AU) and
near-unity mass ratios - are consistent with previous results for field brown
dwarf binaries. Three of the binaries have components that span the
poorly-understood transition between L dwarfs and T dwarfs. Spectral
decomposition analysis of one of these, SDSS 1021-0304AB, reveals a peculiar
flux reversal between its components, as its T5 secondary is ~30% brighter at
1.05 and 1.27 micron than its T1 primary. This system, 2MASS 0518-2828AB and
SDSS 1534+1615AB all demonstrate that the J-band brightening observed between
late-type L to mid-type T dwarfs is an intrinsic feature of this spectral
transition, albeit less pronounced than previously surmised. We also find that
the resolved binary fraction of L7 to T3.5 dwarfs is twice that of other L and
T dwarfs, an anomaly that can be explained by a relatively rapid evolution of
brown dwarfs through the L/T transition, perhaps driven by dynamic
(nonequilibrium) depletion of photospheric condensates.
