Periodic photometric variability of the brown dwarf Kelu-1

We have detected a strong periodicity of 1.80+/-0.05 h in photometric observations of the brown dwarf Kelu-1. The peak-to-peak amplitude of the variation is similar to1.1 per cent (11.9+/-0.8 mmag) in a 41-nm wide filter centred on 857 nm and including the dust/temperature-sensitive TiO and CrH bands. We have identified two plausible causes of variability: surface features rotating in to and out of view and so modulating the light curve at the rotation period; or ellipsoidal variability caused by an orbiting companion. In the first scenario, we combine the observed v sin i of Kelu-1 and standard model radius to determine that the axis of rotation is inclined at 65degrees+/-12degrees to the line of sight.