Variability and rotation in low mass stars and brown dwarfs

The evolution of angular momentum in stars is not really understood today. In particular, very little is known about rotation of substellar objects. Therefore, we started a systematic search for periodic variability in very low mass (VLM) stars and Brown Dwarfs. From a deep multi-filter survey of the young open cluster IC4665, accompanied by follow-up spectroscopy with FORS at the VLT, we selected a field with 190 candidate VLM cluster members. This field was then observed in an extended time series to study photometric variability. We find photometric periods for 13 stars with masses < 0.3 M-circle dot, and for five Brown Dwarfs with masses down to 38 M-Jupiter Furthermore, we find no obvious flares in the time series. These results lead to the conclusion, that VLM objects show less activity and faster rotation than solar mass stars of similar age. For the lowest mass objects the surface features leading to the observed variability may be a kind of clouds from condensated dust in the atmospheres.