We present optical linear polarimetry of 8 ultra cool field dwarfs , with spectral types ranging from M9 to L8 .
The linear polarisation , P , of each dwarf we measured is P < 0.2 % .
Three dwarfs have polarisations compatible with zero , two are marginal detections , and three have significant polarisation .
Due to their small distance , an insterstellar origin for the detected polarisation can be safely ruled out .
Our detections confirm that dust is present in the atmosphere of these brown dwarfs and that the scattering geometry is not symmetric .
Possibilities for asymmetry include the dwarfs rotating rapidly and being oblate , or the cloud coverage in the atmosphere being inhomogenous .