Cosmic microwave background constraints on coupled dark matter
  We study CMB constraints on a model with a cosmological constant and a
fraction of dark matter non-minimally coupled to a massless scalar field. In
this scenario, there is an extra gravity-like fifth force which can affect the
evolution of the Universe enough to have a discernible effect on measurements
of cosmological parameters. Using Planck and WMAP polarisation data, we find
that up to half of the dark matter can be coupled. The coupling can also be
several times larger than in models with a single species of cold dark matter
coupled to a quintessence scalar field, as the scalar field does not play the
role of dark energy and is therefore less constrained by the data.
