  Two classes of stringy instanton effects, stronger than standard field theory
instantons, are identified in the heterotic string theory. These contributions
are established using type IIA/heterotic and type I/heterotic dualities. They
provide examples for the heterotic case of the effects predicted by Shenker
based on the large-order behavior of perturbation theory. The corrections
vanish as the radius of the compactification goes to infinity. For appropriate
amplitudes, they are computable worldsheet or worldline instanton effects on
the dual side. Some potential applications are discussed.
