  In this paper we study a system which is equivalent to a nonlocal version of
the well known Brezis Nirenberg problem. The difficulties related with the lack
of compactness are here emphasized by the nonlocal nature of the critical
nonlinear term. We prove existence and nonexistence results of positive
solutions when $N=3$ and existence of solutions in both the resonance and the
nonresonance case for higher dimensions.
