  Accretion onto black holes often proceeds via an accretion disc or a
temporary disc-like pattern. Variability features, observed in the light curves
of such objects, and theoretical models of accretion flows suggest that
accretion discs are inhomogeneous and non-axisymmetric. Fast orbital motion of
the individual clumps can modulate the observed signal. If the emission from
these clumps is partially polarized, which is likely the case, then rapid
polarization changes of the observed signal are expected as a result of general
relativity effects.
