  Using the Gemini infrared camera on the 3-meter Shane telescope at Lick
Observatory, we have searched for broad-band J and K' photometric variability
for a sample of 15 L and T-type brown dwarfs, including 7 suspected spectral
binaries. Four of the dwarfs - 2MASS J0939-2448, 2MASS J1416+1348A, 2MASS
J1711+2232, and 2MASS J2139+0220 - exhibit statistically significant variations
over timescales ranging from ~0.5 hr to 6 days. Our detection of variability in
2MASS J2139+0220 confirms that reported by Radigan et al., and periodogram and
phase dispersion minimization analysis also confirms a variability period of
approximately 7.6 +/- 0.2 hours. Remarkably, two of the four variables are
known or candidate binary systems, including 2MASS J2139+0220, for which we
find only marginal evidence of radial velocity variation over the course of a
year. This result suggests that some spectral binary candidates may appear as
such due to the blending of cloudy and non-cloudy regions in a single "patchy"
atmosphere. Our results are consistent with an overall variability fraction of
35 +/- 5 %, with no clear evidence of greater variability among brown dwarfs at
the L dwarf/T dwarf transition.
