The pure gauge sector of compactified superstrings

We study the classical field equations of the pure gauge sector of superstrings compactified to four dimensions. Instantons no longer turn out to be solutions of the field equations. We also determine the dynamical behavior of the gauge coupling g in the background of a monopole field, with the result that g and the strong CP-violating angle θ tend to a constant far from the monopole. The energy of such a field configuration is finite.