Observations of ultracool dwarfs with ULTRACAM on the VLT: a search for weather

We present multicolour photometry of four-field ultracool dwarfs with the triple-beam photometer ULTRACAM. Data were obtained simultaneously in the Sloan-g′ band and a specially designed narrow-band Na I filter. The previously reported 1.8-h period of Kelu-1 is here recovered in the g′ band, but the lack of any significant variability in the Na I light of this object precludes any conclusion as to the cause of the variability. 2MASS 2057−0252 and DENIS 1441−0945 show no convincing evidence for variability. 2MASS 1300+1912, on the other hand, shows good evidence for gradual trends in both bands at the 5 per cent level. These trends are anticorrelated at a high level of significance, a result, which is incompatible with models of star-spot induced variability. It would seem likely that dust-cloud ‘weather’ is responsible for the short-term variability in this object.