Cosmological abundances of right-handed Dirac neutrinos

The equilibration of the right-helicity states $\nu_+$ of light Dirac-neutrinos is discussed. I point out that the $\nu_+$ production rate is enhanced by weak gauge boson pole effects so that the right-helicity component of $\nu_\tau$ is brought into equilibrium at $T\simeq 10$ GeV independently of the initial abundance, provided $m_{\nu_{\tau}}\gsim 10$ keV. Neutrino spin flip in primordial magnetic fields and the resulting bound on $\mu_\nu$ is also discussed.