The experimental evidence of neutrino masses and mixing calls for a theoretical framework beyond the Standard Model. We point out that different kinds of new physics may contribute to the observed light neutrino masses. In particular, the seesaw mechanism may be realized via the exchange of two kinds of super-heavy particles: right-handed neutrinos (type I) or isotriplet Higgs bosons (type II). A very special relation connects these two contributions in models with left-right symmetry. In this case, we prove that for each structure of the heavy sector, there exists a dual structure that reproduces the same low energy phenomenology. We explore the consequences of the result for model-building.

SPhT, CEA/Saclay