Constraining the properties of spots on Pleiades very low mass stars

We present results of a multi-filter monitoring campaign for very low mass (VLM) stars in the Pleiades. Simultaneous to our I-band time series (Scholz & Eislöffel 2004b), which delivered photometric periods for nine VLM stars, we obtained light curves in the J- and H-band. One VLM star with M â?? 0.15 Mâ?? (BPL129) shows a period in all three wavelength bands. The amplitudes in I, J, and H are 0.035, 0.035, and 0.032 mag. These values are compared to simulations, in which we compute the photometric amplitude as a function of spot temperature and filling factor. The best agreement between observations and models is found for cool spots with a temperature contrast of 18-31% and a very low surface filling factor of 4-5%. We suggest that compared to more massive stars VLM objects may have either very few spots or a rather symmetric spot distribution. This difference might be explained with a change from a shell to a distributed dynamo in the VLM regime. © ESO 2005.