  We consider conformal vector models which could play the role of a
cosmological dark radiation component. We analyse the propagation of
gravitational waves in the presence of this vector background and find a
suppression in the tensor transfer function at large scales. We also find that
although the cosmological background metric is isotropic, anisotropies are
imprinted in the tensor power spectrum. In addition, the presence of the
background vector fields induces a net polarization of the gravitational wave
background and, for certain configurations of the vector field, a linear to
circular polarization conversion. We also show that this kind of effects are
also present for vector models with more general potential terms.
