Optical polarisation variability of radio loud narrow line Seyfert 1
  galaxies. Search for long rotations of the polarisation plane
  Narrow line Seyfert 1 galaxies (NLSy1s) constitute the AGN subclass
associated with systematically smaller black hole masses. A few radio loud ones
have been detected in MeV -- GeV energy bands by Fermi and evidence for the
presence of blazar-like jets has been accumulated. In this study we wish to
quantify the temporal behaviour of the optical polarisation, fraction and
angle, for a selected sample of radio loud NLSy1s. We also search for rotations
of the polarisation plane similar to those commonly observed in blazars. We
have conducted R-band optical polarisation monitoring of a sample of 10 RL
NLSy1s 5 of which have been previously detected by Fermi. The dataset includes
observations with the RoboPol, KANATA, Perkins and Steward polarimeters. In the
cases where evidences for long rotations of the polarisation plane are found,
we carry out numerical simulations to assess the probability that they are
caused by intrinsically evolving EVPAs instead of observational noise. Even our
moderately sampled sources show indications of variability, both in
polarisation fraction and angle. For the four best sampled objects in our
sample we find multiple periods of significant polarisation angle variability.
In the two best sampled cases, namely J1505+0326 and J0324+3410, we find
indications for three long rotations. We show that although noise can induce
the observed behaviour, it is much more likely that the apparent rotation is
caused by intrinsic evolution of the EVPA. To our knowledge this is the very
first detection of such events in this class of sources. In the case of the
largest dataset (J0324+3410) we find that the EVPA concentrates around a
direction which is at 49.3\degr to the 15-GHz radio jet implying a projected
magnetic field at an angle of 40.7\degr to that axis.
