  We suggest a perturbative approach for generic choices for the universe
equation of state and introduce a novel framework for studying mass varying
neutrinos (MaVaN's) coupled to the dark sector. For concreteness, we examine
the coupling between neutrinos and the underlying scalar field associated with
the generalized Chaplygin gas (GCG), a unification model for dark energy and
dark matter. It is shown that the application of a perturbative approach to
MaVaN mechanisms translates into a constraint on the coefficient of a linear
perturbation, which depends on the ratio between a neutrino energy dependent
term and scalar field potential terms. We quantify the effects on the MaVaN
sector by considering neutrino masses generated by the seesaw mechanism. After
setting the GCG parameters in agreement with general cosmological constraints,
we find that the squared speed of sound in the neutrino-scalar GCG fluid is
naturally positive. In this scenario, the model stability depends on previously
set up parameters associated with the equation of state of the universe. Our
results suggest that the GCG is a particularly suitable candidate for
constructing a stable MaVaN scenario.
