  We revisit the process of inverse neutrino-less double beta decay (e e -> W
W) at future linear colliders. The cases of Majorana neutrino and Higgs triplet
exchange are considered. We also discuss the processes e mu -> W W and mu mu ->
W W, which are motivated by the possibility of muon colliders. For heavy
neutrino exchange and center-of-mass energies larger than 1 TeV, we show that
masses up to 10^6 (10^5) GeV could be probed for e-e and e-mu machines,
respectively. The stringent limits for mixing of heavy neutrinos with muons
render mu mu -> W W less promising, even though this process is not constrained
by limits from neutrino-less double beta decay. If Higgs triplets are
responsible for inverse neutrino-less double beta decay, observable signals are
only possible if a very narrow resonance is met. We also consider unitarity
aspects of the process in case both Higgs triplets and neutrinos are exchanged.
An exact see-saw relation connecting low energy data with heavy neutrino and
triplet parameters is found.
