  We present a low energy model for the Bose-Einstein condensation in a
quasi-two-dimensional excitonic gas. Using the flow equations of the
Renormalization group and a $\Phi^4$ model with the dynamical critical exponent
$z=2$ we calculate the temperature dependence of the critical density,
coherence length, magnetic susceptibility, and specific heat. The model can be
relevant for the macroscopic coherence observed in GaAs/AlGaAs coupled quantum
wells.
