  We report differential I-band and J-band photometry of S Ori 45, a cool
(M8.5), young (1-8 Myr) brown dwarf of the sigma Orionis cluster with a likely
mass estimated at around 20 times the mass of Jupiter. We detect variability
(amplitudes ranging from 34 to 81 mmag) and observe a modulation at a period of
2.5-3.6 h in both optical and near-infrared light curves. The most recent
optical data set, however, presents a modulation at the very short period of
46.4+/-1.5 min, which remains a mystery. The origin of the 2.5-3.6 h modulation
is analized in terms of various scenarios: inhomogeneous features (dust clouds
or magnetically induced dark spots) co-rotating with the object's surface, and
presence of an unseen very low-mass companion that is steadily transferring
mass to the primary. Because of the very young age of the object and its
persistent strong Halpha emission, the possible presence of an accreting disk
is also discussed. If the period of a few hours is related to rotation, our
results suggest that sigma Orionis low-mass brown dwarfs are rotating faster
than more massive cluster brown dwarfs at a rate consistent with their
theoretically inferred masses and radii, implying that all of these objects
have undergone similar angular momentum evolution.
