  We study the `asymmetric' Hubbard model, where hoppings of electrons depend
on their spin. For strong interactions and sufficiently asymmetric hoppings, it
is proved that the ground state displays phase separation away from
half-filling. This extends a recent result obtained with Freericks and Lieb for
the Falicov-Kimball model. It is based on estimates for the sum of lowest
eigenvalues of the discrete Laplacian in arbitrary domains.
