Linear polarization can be used as a probe of the existence of atmospheric condensates in ultracool dwarfs .
Models predict that the observed linear polarization increases with the degree of oblateness , which is inversely proportional to the surface gravity .
We aimed to test the existence of optical linear polarization in a sample of bright young brown dwarfs , with spectral types between M6 and L2 , observable from the Calar Alto Observatory , and cataloged previously as low gravity objects using spectroscopy .
Linear polarimetric images were collected in I and R -band using CAFOS at the 2.2 m telescope in Calar Alto Observatory ( Spain ) .
The flux ratio method was employed to determine the linear polarization degrees .
With a confidence of 3 \sigma , our data indicate that all targets have a linear polarimetry degree in average below 0.69 % in the I -band , and below 1.0 % in the R -band , at the time they were observed .
We detected significant ( i.e . P / \sigma \geq 3 ) linear polarization for the young M6 dwarf 2MASS J04221413 + 1530525 in the R -band , with a degree of p ^ { * } = 0.81 \pm 0.17 % .