  A coupling between a light scalar field and neutrinos has been widely
discussed as a mechanism for linking (time varying) neutrino masses and the
present energy density and equation of state of dark energy. However, it has
been pointed out that the viability of this scenario in the non-relativistic
neutrino regime is threatened by the strong growth of hydrodynamic
perturbations associated with a negative adiabatic sound speed squared. In this
paper we revisit the stability issue in the framework of linear perturbation
theory in a model independent way. The criterion for the stability of a model
is translated into a constraint on the scalar-neutrino coupling, which depends
on the ratio of the energy densities in neutrinos and cold dark matter. We
illustrate our results by providing meaningful examples both for stable and
unstable models.
