  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.
