Cladis' orbiting disclinations in smectic films submitted to a torque

We study here the behaviour of S=+1 and S=-1 disclinations in SmC films sub mitted to a rotating electric field. In agreement with Cladis et ai. experi ments we observe that a S=+1 disclination submitted to large enough torque leaves the centre of the film and is orbiting around a target-like pattern. A single S=-1 disclination submitted to a torque behaves differ entry: ind ependently of the applied torque, it stays in the centre of the film. We ex plain this difference as due to the presence (S=+1) and the absence (S=-1) of energy barriers with respect to the phase winding.