  We investigate the decoupling of heavy right handed neutrinos in the context
of an SO(10) GUT model, where a remnant anomalous symmetry is $U(1)_{X}$. In
this model the see-saw mechanism which generates the neutrino masses is
intertwined with the Stueckelberg mechanism, which leaves the CP-odd phase of a
very heavy Higgs in the low energy spectrum as an axion-like particle. Such
pseudoscalar is predicted to be ultralight, in the $10^{-20}$ eV mass range. In
this scenario, the remnant anomalous $X$ symmetry of the particles of the
Standard Model is interpreted as due to the incomplete decoupling of the right
handed neutrino sector. We illustrate this scenario including its realisation
in the context of SO(10).
