  We show that different non-conventional superconductors have one fundamental
feature in common: pair eigenstates of the Hamiltonian are repulsion-free, the
W=0 pairs. In extended Hubbard models, pairing can occur for resonable
parameter values. For $(N,N)$ nanotubes the binding energy of the pair depends
strongly on the filling and decreases towards a reduced but nonzero value for
the graphite sheet $N \to \infty$.
