  Field theories with p-form gauge potentials can possess ``hidden'' symmetries
leaving the field strengths invariant on-shell without being gauge symmetries
on-shell. The relevance of such symmetries to supersymmetric models is
discussed. They provide central charges of supersymmetry algebras, play a
particular role in duality relations, and lead to peculiar interactions. A
multiplet of N=2 supersymmetry in four dimensions with two hidden central
charges is presented.
