Multicolour time series photometry of three periodic ultracool dwarfs

Photometry in I, or contemporaneously in I and R, of the known variable ultracool dwarfs Kelu-1 and 2MASS J11553952−3727350 is presented. The nature of the variability of Kelu-1 appears to evolve on time-scales of a day or less. Both the period and amplitude of the variability of 2MASS J11553952−3727350 have changed substantially since publication of earlier observations of the object. DENIS 1454−6604 is a new variable ultracool dwarf, with persistent and prominent brightness modulations at a period of 2.6 h.