  We discuss the fixed-point Hamiltonian and the spectrum of excitations of a
quasi-bidimensional electronic system supporting simultaneously
antiferromamagnetic ordering and superconductivity. The coexistence of these
two order parameters in a single phase is possible because the magnetic order
is linked to the formation of a spin density wave, and its order parameter is
not associated to a spectral gap but to an energy shift of the paramagnetic
bands. This peculiarity entails several distinct features in the phase diagram
and the spectral properties of the model, which may have been observed in
CeRhIn$_5$. Apart from the coexistence, we find an abrupt suppression of the
spin density wave when the superconducting and magnetic ordering temperatures
are equal. The divergence of the cyclotron mass extracted from de Haas-van
Alphen experiments is also analyzed in the same framework.
