Towards necessary and sufficient conditions for halo occurrence

We define nuclear halos states and derive and discuss occurrence conditions leading to constraints on neutron N and proton Z numbers, the total nuclear excitation energy E∗ and the binding energy B of the halo state. Only extremely small values of E∗ allow halo formation, which also requires small B. Low N and Z seems to be favored. We give upper limits for B for E∗=0.