  We discuss the process $e^-e^- \to W^- W^-$ mediated by heavy Majorana
neutrino exchange in the t- and u channel. In our model the cross section for
this reaction is a function of the masses (m_N) of the heavy Majorana neutrinos
and mixing parameters (U_{eN}) originating from mixing between the ordinary
left-handed standard model neutrinos and additional singlet right-handed
neutrino fields. Taking into account the standard model background and
contraints from low energy measurements, we present discovery limits in the
(m_N,U_{eN}^2) plane. We also discuss how to measure in principle the CP
violating phases, i.e., the relative phases between the mixing parameters.
