COSMOLOGICAL BOUNDS ON DIRAC-MAJORANA NEUTRINOS

We examine, by way of a detailed numerical calculation, the consequences for primordial nucleosynthesis of a weakly interacting neutrino which possesses both Dirac and Majorana mass terms. In the special limiting case of a pseudo-Dirac neutrino we place bounds on the pseudo-Dirac mass splitting delta-m(p-D). This bound is relevant to a class of models constructed to incorporate the proposed 17 keV neutrino. We find that in the standard model \delta-m(p-D)\ < 10(-10) eV. In the models with majorons the bound is less restrictive, but still several orders of magnitude more stringent than previous estimates.