  Numerical and analytical studies of several models of correlated electrons
are discussed. Based on exact diagonalization and variational Monte Carlo
techniques, we have found strong indications that the two dimensional t-J model
superconducts near phase separation in the regime of quarter-filling density,
in agreement with previous results reported by Dagotto and Riera (Phys. Rev.
Letters 70, 682 (1993)). At this density the dominant channel is d_{x^2-y^2},
but a novel transition to s-wave superconductivity is observed decreasing the
electronic density. In addition, the one band t-U-V model has also been studied
using the mean-field approximation that accurately described the spin density
wave phase of the repulsive Hubbard model at half-filling. Finally, the two
band Hubbard model on a chain is also analyzed. Superconducting correlations
near phase separation exist in this model, as it occurs in the t-J model. Based
on these nontrivial examples it is $conjectured$ that electronic models tend to
have superconducting phases in the vicinity of phase separation.Reciprocally,
if it is established that a model that does not phase separate, then its
chances of presenting a superconducting phase are considerably reduced.
