  We determine the restrictions imposed by primordial nucleosynthesis upon a
heavy tau neutrino, in the presence of nu-tau annihilations into Majorons, as
expected in a wide class of particle physics models of neutrino mass. We
determine the equivalent number of light neutrino species $N_{eq}$ as a
function of nu-tau mass and the nu-tau-Majoron coupling $g$. We show that for
theoretically plausible $g$ values $\gsim 10^{-4}$ present nucleosynthesis
observations can not rule out nu-tau masses in the MeV range. Moreover, these
models give $N_{eq} \leq 3$ in the nu-tau mass region 1-10 MeV, for very
reasonable values of $g \geq 3 \times 10^{-4}$. The evasion of the cosmological
limits brings new interest to the improvement of the present laboratory limit
on the nu-tau mass which can be achieved at a tau-charm factory.
