  We discuss the implications on light by light scattering of two kind of
exotic particles: doubly charged scalar bosons and doubly charged fermions; the
virtual effects of a nonstandard singly charged gauge boson are also examined.
These particles, if their masses lie in the range 0.1--1.0 TeV, will have a
clear signature in the future linear colliders. The present analysis has the
advantage that it depends only on electromagnetic symmetry, so it is applicable
to any model which predicts this class of particles. In particular, our results
have interesting consequences on left-right models and their supersymmetric
extension.
