S-duality improved perturbation theory in compactified type I/heterotic string theory

We study the mass of the stable non-BPS state in type I/heterotic string theory compactified on a circle with the help of the interpolation formula between weak and strong coupling results. Comparison between the results at different orders indicate that this procedure can determine the mass of the particle to within 10% accuracy over the entire two dimensional moduli space parametrized by the string coupling and the radius of compactification. This allows us to estimate the region of the stability of the particle in this two dimensional moduli space. Outside this region the particle is unstable against decay into three BPS states carrying the same total charge as the original state. We discuss generalization of this analysis to compactification on higher dimensional tori.