  We study the effects induced by new neutral fermions below their mass
threshold, due to their possible mixing with the standard neutrinos. We use as
experimental constraints the recent results on lepton universality, together
with the measurement of the $\mu$ decay rate and the updated LEP data. In
particular, the inclusion in our data set of the most recent determinations of
the $\tau$ branching fractions, mass and lifetime implies that a previous
indication of a non-vanishing mixing for $\nu_\tau$ is no longer present. We
obtain new stringent limits on the mixing parameters between $\nu_e$,
$\nu_\mu$, $\nu_\tau$ and heavy neutral states of different weak isospin. If no
assumption on the type of neutrinos involved in the mixing is made, we find
$\snue^2<0.0071$, $\snumu^2<0.0014$ and $\snutau^2<0.033$.
