OPEP and involves an intermediate pion-nucleon ($\pi N$) amplitude in a kinematical region which is not directly accessible to experiment. Therefore a proper theoretical treatment of this component of the potential requires the use of information based on dispersion relations, as pointed out long ago by Cottinghan and VinMau [@Vin+]. The implementation of this idea led to the Paris potential [@Lac+75], which is rather successful in describing experimental data.

An important feature of the $\pi N$ amplitude is chiral symmetry, as emphasized more than 25 years ago by Brown and Durso [@Bro+]. Recently the application of chiral symmetry to the NN interaction has deserved much attention, initially in the restricted framework of pion and nucleon degrees of freedom [@chi], and nowadays it is fair to say that the theoretical formulation of this sector of the interaction is free of ambiguities. However, a Lagrangian containing only pions and nucleons cannot reproduce low-energy $\pi$N data and hence this very reliable part of the model has to be complemented [@Kol+; @Rob+]. The combination of chiral symmetry and experimental information about the intermediate $\pi N$ amplitude, based on dispersion relations, was brought into this problem in the last two years [@Rob+], with a successful 