  Minimal extensions of the Standard Model that are motivated by grand unified
theories or superstring models with an E_6 symmetry can naturally predict heavy
neutrinos of Dirac or Majorana nature. Such heavy neutral leptons violate the
de- coupling theorem at the one-loop electroweak order and hence offer a unique
chance for possible lepton-flavour decays of the tau lepton, e.g. tau -> eee or
tau -> mu mu mu, to be seen in LEP experiments. We analyze such decays in
models with three and four generations.
