  We propose a canonical tranformation approach to the effective interaction
$W_{eff}$ between two holes, based on the three-band Hubbard model but ready to
include extra interactions as well. An effective two-body Hamiltonian can in
principle be obtained including any kind of virtual intermediate states. We
derive the closed-form analytic expression of the effective interaction
including 4-body virtual states, describing the exchange of an electron-hole
pair to all orders. The resulting integral equation, valid for the full plane,
leads to a Cooper-like instability of the Fermi liquid. The two-hole bound
states generalize those reported earlier in cluster calculations by exact
diagonalisation methods.
