We would like to thank Prof. John Cornwall for very interesting comments and remarks about this work and Prof. Zvi Bern and Prof. P. Gaete for conversations about the topics disussed here. We also thank the referee for rising some points concerning the relation between the results obtained in this paper and the ones already present in the literature on the subject and for helping us, with his/her observations, in finding a mistake in the first version of the paper. One of us, E.I.G., would like to thank University of Trieste and U.C.L.A. for hospitality and I.N.F.N. for support.