Non-adiabatic instability in coupled dark sectors
  It has been recently pointed out that coupled dark matter-dark energy systems
suffer from non-adiabatic instabilities at early times and large scales. We
show how coupled models free from non-adiabatic instabilities can be identified
as a function of a generic coupling Q and of the dark energy equation of state
w. In our analysis, we do not refer to any particular cosmic field. We also
confront a viable class of model in which the interaction is directly
proportional to the dark energy density to recent cosmological data. In that
framework, we show the correlations between the dark coupling and several
cosmological parameters allowing to e.g.larger neutrino mass than in uncoupled
models.
