  We show that the cross section of at least one of the s-channel processes
e-e- (mu-mu-) --> l_i^- l_i^-, i=e,mu,tau, mediated by a doubly-charged scalar
triplet bilepton is bounded from below and observable at a linear or muon
collider if one of the light neutrinos has a mass in the range where it is
required to be unstable by cosmological considerations. The result is model
independent. We therefore stress the importance of the e-e- and mu-mu-
collision modes of the future colliders for discovering new physics.
