A search for variability in brown dwarfs and L dwarfs

We have undertaken a differential photometric I band search for variability in three Pleiades brown dwarfs and three very low mass field L dwarfs. Analysis of the resultant time series for the Pleiades brown dwarfs (Teide 1, Calar 3, Roque 11) gives no evidence for variability (above 99% confidence) greater than 0.05 mags in any of these objects on timescales between 25 minutes and 27 hours. Time series of two of the L dwarfs, 2MASSW J0913032+184150 and 2MASSW J1146345+223053, are also consistent with no intrinsic variability (no significant variability above 0.07 mags for 30 mins < t < 126 hours, and above 0.025 mags for 30 mins < t < 75 hours, for the two objects respectively). However, the L dwarf 2MASSW J1145572+231730 shows evidence for variability of amplitude 0.04 magnitudes with a tentative period of about 7 hours. It is therefore interesting that Kirkpatrick et al. (1999) report an Halpha emission line of 4.2AA equivalent width for this star: If our detection is confirmed as a rotation period, it would support the link between rotation and Halpha emission in very low mass stars.