  Naturally small neutrino masses can arise in some grand unified models. The
mechanism of neutrino mass generation in these models typically requires the
existence of neutral heavy leptons. We study the low-energy phenomenology of
these new fermions. Concentrating on loop corrections due to neutral heavy
leptons, we examine how the flavour-conserving leptonic decays of the Z boson,
universality breaking in these decays, and the W boson mass depend on the mass
and mixings of the neutral heavy leptons. Working within the framework of a
superstring-inspired SU(2)_L x U(1)_Y model, we show that these
flavour-conserving processes have some virtues over the traditionally
considered flavour-violating decays.
