  We study a dark energy model with non-zero anisotropic stress, either linked
to the dark energy density or to the dark matter density. We compute
approximate solutions that allow to characterise the behaviour of the dark
energy model and to assess the stability of the perturbations. We also
determine the current limits on such an anisotropic stress from the cosmic
microwave background data by the Planck satellite, and derive the corresponding
constraints on the modified growth parameters like the growth index, the
effective Newton's constant and the gravitational slip.
