  We determine the range of parameter space of an interacting quintessence (IQ)
model that best fits the luminosity distance of type Ia supernovae data and the
recent WMAP measurements of Cosmic Microwave Background temperature
anisotropies. Models in which quintessence decays into dark matter provide a
clean explanation for the coincidence problem. We focus on cosmological models
of zero spatial curvature. We show that if the dark energy (DE) decays into
cold dark matter (CDM) at a rate that brings the ratio of matter to dark energy
constant at late times, the supernovae data are not sufficient to constrain the
interaction parameter. On the contrary, WMAP data constrain it to be smaller
than $c^2 < 10^{-2}$ at the $3\sigma$ level. Accurate measurements of the
Hubble constant and the dark energy density, independent of the CMB data, would
support/disprove this set of models.
