  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.

High Energy Physics - Phenomenology