on the imaginary part of the relevant combinations of neutrino Yukawa couplings are obtained and plotted in the plane $(\ti M_1, m_R)$, respectively the bino and the average R-slepton masses at low energy. These plots allow to check whether any particular seesaw model is consistent with present data and, if so, which level of experimental sensitivity would test it. In any case, an experimentally interesting contribution requires hierarchical right-handed neutrino masses.

This feature is no more necessary when, in addition to the seesaw, a stage of $SU(5)$ grand unification is present above the gauge coupling unification scale. It is well known that the main drawback of minimal $SU(5)$ is that the value of the triplet mass, $M_T$, required by gauge coupling unification is sizeably below the lower bound on $M_T$ derived from proton lifetime (see for instance [@isa_ijmpa] for recent reviews and references). In our analysis we therefore keep the triplet mass as a free parameter. We nevertheless exploit the minimal $SU(5)$ relations between the doublet and triplet Yukawa couplings since in general they are mildly broken in non-minimal versions of $SU(5)$. The simultaneous presence of right-handed neutrinos and heavy triplets turns out to further enhance the amplitude with the 