Coulomb effects on single-particle exchange singularities

We consider the singularity in the scattering amplitude arising from the exchange of a massive neutral particle. It is shown that the presence of Coulomb distortion in the entrance or exit channel modifies the exchange pole residue. Within the framework of the Coulomb wave Born approximation the exchange pole residue is modified by an energy-dependent factor that is independent of the orbital angular momentum of the bound states involved in the exchange process. The implications of this result on the polarization observables and on recent determinations of the deuteron asymptotic D- to S-state ratio, by analytic continuation in cos θ to the neutron exchange pole, are discussed.