Rapid optical variability in ultracool dwarfs

The observing strategy was to monitor each object continuously for 2-3h in order to ascertain whether it was rapidly variable. Many of the targets were observed at multiple epochs, to follow up possible short time-scale variability, or to test for slow brightness changes on longer time-scales.

The 353 data sets obtained contain nearly 22000 individual measurements. Optical (I_C_) magnitudes, accurate to roughly 0.1-0.2mag, were derived for 21 objects for which there is no optical photometry in the literature. It is shown that photometry is affected by variable seeing in a large percentage of the time-series observations.