  In this paper, we study the phenomenology of right-handed neutrino
isosinglets. We consider the general situation where the neutrino masses are
not necessarily given by $m_D^2/M$, where $m_D$ and $M$ are the Dirac and
Majorana mass terms respectively. The consequent mixing between the light and
heavy neutrinos is then not suppressed, and we treat it as an independent
parameter in the analysis. It turns out that $\mu-e$ conversion is an important
experiment in placing limits on the heavy mass scale ($M$) and the mixing.
Mixings among light neutrinos are constrained by neutrinoless double beta
decay, as well as by solar and atmospheric neutrino experiments. Detailed
one-loop calculations for lepton number violating vertices are provided.
