• techniques: photometric;
  • stars: atmospheres;
  • binaries: close;
  • stars: low-mass, brown dwarfs;
  • stars: oscillations;
  • stars: rotation

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 ∼1.1 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 65°±12° to the line of sight.