Heavy Majorana neutrinos at e+e- colliders

We investigate possibilities for detecting heavy Majorana neutrinos ($N$'s) in $e^+ e^-$ at LEP200 and future Linear Colliders. We concentrate on the processes where the pairs of intermediate heavy $N$'s produce a clear signal of total lepton number violation ($e^+ e^- \to N N \to W^+ l^- W^+ l^{\prime -}$). Such a signal is not possible if the heavy neutrinos are of Dirac nature. Our approach is general in the sense that the intermediate $N$'s can be either on-shell or off-shell. Discussion of the relative numerical importance of the $s$ and the $t+u$ channels of the $NN$ production is also included.