  New heavy Majorana and Dirac neutrinos production at future electron-muon
colliders are investigated. The production of a single heavy neutrino is shown
to be more relevant than pair production when comparing cross sections and
neutrino mass ranges. The process $e^\pm \mu^\mp \longrightarrow {\nu}
\ell^{\pm} W^{\mp}$ is studied including on-shell and off-shell heavy neutrino
effects. Distributions are calculated including hadronization effects and
experimental cuts that suppress background, in order to have a clear signal for
heavy neutral leptons.
