  Cosmological perturbation equations derived from low-energy effective actions
are shown to be invariant under a duality transformation reminiscent of
electric-magnetic, strong-weak coupling, S-duality. A manifestly
duality-invariant approximation for perturbations far outside the horizon is
introduced, and it is argued to be useful even during a high curvature epoch.
Duality manifests itself through a remnant symmetry acting on the classical
moduli of cosmological models, and implying lower bounds on the number and
energy density of produced particles.
