We present here observations of the polarisation of four auroral lines in the auroral oval and in the polar cusp using a new ground polarimeter called <i>Petit Cru</i>. Our results confirm the already known polarisation of the red line, and show for the first time that the three other lines observed here (namely 557.7 nm, 391.4 nm and 427.8 nm) are polarised as well up to a few percent. We show that ...