  We have resolved the newly discovered T dwarf SDSS J1534+1615 into a 0.11''
binary using the Keck sodium laser guide star adaptive optics system. With an
integrated-light spectral type of T3.5, this binary provides a new benchmark
for studying the distinctive J-band brightening previously noted among early
and mid-T dwarfs, using two brown dwarfs with different spectral types but
having a common metallicity and age and very similar surface gravities. We
estimate spectral types of T1.5+/-0.5 and T5.5+/-0.5 for the two components
based on their near-IR colors, consistent with modeling the integrated-light
spectrum as the blend of two components. The observed near-IR flux ratios are
unique compared to all previously known substellar binaries: the component that
is fainter at H and K' is brighter at J. This inversion of the near-IR fluxes
is a manifestation of the J-band brightening within this individual binary
system. Therefore, SDSS 1534+1615 demonstrates that the brightening can be
intrinsic to ultracool photospheres (e.g., arising from cloud disruption and/or
rapid increase in cloud sedimentation) and does not necessarily result from
physical variations among the observed ensemble of T dwarfs (e.g., a range in
masses, ages and/or metallicities). We suggest that the apparently large
amplitude of the J-band brightening may be due to a high incidence of
unresolved binaries and that the true amplitude of the phenomenon could be more
modest. This scenario would imply that truly single objects in these spectral
subclasses are relatively rare, in agreement with the small effective
temperature range inferred for the L/T transition.
