  Aims.Recent studies have detected linear polarization in L dwarfs in the
optical I band. Theoretical models have been developed to explain this
polarization. These models predict higher polarization at shorter wavelengths.
We discuss the polarization in the R and I band of 4 ultra cool dwarfs.
Methods.We report linear polarization measurements of 4 ultra cool dwarfs in
the R and I bands using the Intermediate dispersion Spectrograph and Imaging
System (ISIS) mounted on the 4.2m William Herschel Telescope (WHT). Results.As
predicted by theoretical models, we find a higher degree of polarization in the
R band when compared to polarization in the I band for 3/4 of these ultra cool
dwarfs. This suggests that dust scattering asymmetry is caused by oblateness
>.We also show how these measurements fit the theoretical models. A case for
variability of linear polarization is found, which suggests the presence of
randomly distributed dust clouds. We also discuss one case for the presence of
a cold debris disk.
