  The Josephson transport and the electronic properties of a quantum dot
characterized by a single level coupled to superconducting leads is analyzed.
Different approximations are used and compared: the mean field approximation,
the second order perturbation theory in the Coulomb interaction and the exact
diagonalization in the zero band-width limit. The system exhibits a rich
behavior as a function of the relevant parameters. We discuss in detail the
conditions for the observation of Pi-junction behavior and the effect of
Coulomb interactions on the Andreev states.
