  We investigate the decays $l_i\rightarrow l_j \gamma$, with $l_i=e,\mu,\tau$
in a general class of 3-3-1 models with heavy exotic leptons with arbitrary
electric charges. We present full and exact analytical results keeping external
lepton masses. As a by product, we perform numerical comparisons between exact
results and approximate ones where the external lepton masses are neglected. As
expected, we found that branching fractions can reach the current experimental
limits if mixings and mass differences of the exotic leptons are large enough.
We also found unexpectedly that, depending on the parameter values, there can
be huge destructive interference between the gauge and Higgs contributions when
the gauge bosons connecting the Standard Model leptons to the exotic leptons
are light enough. This mechanism should be taken into account when using
experimental constraints on the branching fractions to exclude the parameter
space of the model.
