which is the chiral partner of the pion. The value $\lambda=1$, in turn, indicates processes based on a chiral invariant field S. It is worth pointing out that results with $\lambda=1$ are, as they should, independent of the representation adopted for the pion field and, in particular, of the use of either PS or PV pion-nucleon couplings. So the parameter $\lambda$ describes dynamical processes which are genuinely different.

At tree level, these effects cannot be distinguished in NN interactions, but they manifest themselves in the reactions NN $\rightarrow\pi$NN, $\pi$d$\leftrightarrow$NN, in axial form factors of nuclei and in three-body forces. In the present work we have considered only the last kind of application.

Our results with $\lambda=1$ coincide with that presented by Coon, Peña and Riska [@CPR]. These authors also obtained a contact interaction, employing a PV pion-nucleon coupling and a scalar-meson coupled to nucleons. In the framework of chiral symmetry, this combination means that they have tacitly used a Lagrangian equivalent of our eq.(5) and hence their $\sigma$ field corresponds to our S.

In this work we are concerned mainly with the differences between the cases $\lambda=0$ and $\lambda=1$ for three-body forces. The full exploration of this aspect of the 