Inflation in S-Dual Superstring Models

We study inflationary potentials in the framework of superstring theories. Successful inflation may occur due to chiral fields, but only after the dilaton and moduli are stabilized. This is achieved by demanding an S-duality invariant potential. Then, it is possible to have inflation at any scale and even to have more than one stages of inflation. This occurs for a limited class of scalar potentials, where certain conditions are obeyed. Density fluctuations of $10^{-5}$ require the inflationary potential to be at a scale of $V^{1/4}\simeq 5 \times 10^{15}$ GeV.