  We use a D-instanton or physical gauge approach to re-derive the heterotic
string worldsheet instanton contribution to the superpotential in Calabi-Yau
compactification. We derive an analogous formula for worldsheet instanton
corrections to the moduli space metric in heterotic string or Type I
compactification on a K3 surface. In addition, we give a global analysis of the
phase of the worldsheet path integral of the heterotic string, showing
precisely how the B-field must be interpreted.
