  We report the discovery of a photometric variability in the bright T2.5 brown
dwarf SIMP J013656.5+093347. Continuous J-band photometry has been obtained for
several hours on four different nights. The light curves show a periodic
modulation with a period of ~2.4 hours, a peak-to-peak amplitude of ~50 mmag
and significant night-to-night evolution. We suggest that the light curve
modulation is due to the brown dwarf's rotation and that the longer term
variations come from surface features evolution and/or differential rotation.
We obtained complementary observations over a single night in the J and Ks
bands; the object displays correlated photometric variability in both bands,
albeit with smaller Ks-band amplitude. The ratio of the Ks and J variability
amplitudes puts strong constraints on the physical mechanisms at play. Based on
theoretical models of brown dwarf atmospheres, our results suggest that the
atmosphere of SIMP0136 is comprised of both grain-free and colder (by ~100 K)
grain-bearing cloudy regions. This discovery, and its interpretation, provide a
natural explanation of the so-called J-band brightening.
