  Models where the dark matter component of the universe interacts with the
dark energy field have been proposed as a solution to the cosmic coincidence
problem, since in the attractor regime both dark energy and dark matter scale
in the same way. In these models the mass of the cold dark matter particles is
a function of the dark energy field responsible for the present acceleration of
the universe, and different scenarios can be parameterized by how the mass of
the cold dark matter particles evolves with time. In this article we study the
impact of a constant coupling $\delta$ between dark energy and dark matter on
the determination of a redshift dependent dark energy equation of state
$w_{DE}(z)$ and on the dark matter density today from SNIa data. We derive an
analytical expression for the luminosity distance in this case. In particular,
we show that the presence of such a coupling increases the tension between the
cosmic microwave background data from the analysis of the shift parameter in
models with constant $w_{DE}$ and SNIa data for realistic values of the present
dark matter density fraction. Thus, an independent measurement of the present
dark matter density can place constraints on models with interacting dark
energy.
