Abstract. We study an elliptic system of the form Lu = |v| p−1 v and Lv = |u| q−1 u in Ω with homogeneous Dirichlet boundary condition, where Lu := −Δu in the case of a bounded domain and Lu := −Δu + u in the cases of an exterior domain or the whole space R N . We analyze the existence, uniqueness, sign and radial symmetry of ground state solutions and also look for sign changing solutions of the system. More general non-linearities are also considered.