An improved cosmological bound on the tau-neutrino mass

We consider the influence of non-equilibrium electronic neutrinos (and anti-neutrinos) on the neutron-to-proton ratio. These neutrinos would come from massive $\nu_\tau$ annihilations $\bar \nu_\tau \nu_\tau \rightarrow \bar \nu_e \nu_e$. For sufficiently large $\nu_\tau$ masses this new effect would strongly enhance the (n/p)-ratio, leading to a very stringent bound on the $\nu_\tau$ mass, even adopting a rather weak upper bound on the effective number on neutrino species during nucleosynthesis.