  The problem of two coupled scalar fields, one with mass much lighter than the
other is analysed by means of Wilson's renormalization group approach. Coupled
equations for the potential and the wave function renormalization are obtained
by means of the gradient expansion. The Appelquist-Carazzone theorem is checked
at energies comparable with the light mass; the physics of the light field is
also studied at energies comparable and greater than the heavy mass.
