A search for short time-scale JHK variability in ultracool dwarfs

The results of continuous monitoring, with typical durations of 2-4 h, of 18 L and T dwarfs are presented. Measurements were made simultaneously in JHK(S), with a typical time resolution of about 10 min. Limits on the variability are well below 0.02 mag in all cases except for the faintest object. There are none the less a fewdwarfs in which we may have observed variability: the same patterns of very low-level activity seemed to be present in the data for more than one filter. Simultaneous frequency-domain analysis of the J, H and K-S data furthermore hints at a very short period (0.8-1.6 h) in a few objects. We also present a brief assessment of some of the variability test statistics that have been used in the field of ultracool dwarf variability.