Analytic expressions of lepton-flavour- and lepton-number-violating de
cays of charged leptons are derived in the context of general SU(2)(L)
x U(1)(g)amma seesaw scenarios that are motivated by grand unified th
eories (GUT's) or superstring models, in which left-handed and/or righ
t-handed neutral singlets are present. Possible constraints imposed by
cosmology and low-energy data are briefly discussed. The violation of
the decoupling theorem in flavour-dependent graphs due to the presenc
e of heavy neutral leptons of Dirac or Majorana nature is emphasized.
Numerical estimates reveal that the decays tau(-) --> e(-)e(-)e(+) or
tau --> e(-)mu(-)mu(+) can be as large as similar to 10(-6), which may
be observed in LEP experiments or other tau factories.