Charged string solutions with dilaton and modulus fields

We find charged, abelian, spherically symmetric solutions (in flat space-time) corresponding to the effective action of D=4 heterotic string theory with the scale-dependent dilaton Φ and modulus φ fields. We take into account perturbative (genus-one), moduli-dependent ''threshold'' corrections to the coupling function f(Φ, φ) in the gauge field kinetic term f(Φ, φ)F2μν, as well as the non-perturbative scalar potential V(Φ, φ), e.g., induced by gaugino condensation in the hidden gauge sector. Stable, finite-energy, electric solutions (corresponding to an abelian subgroup of a non-abelian gauge group) have the small scale region as the weak coupling region (Φ→∞) with the modulus φ slowly varying towards smaller values. Stable, finite-energy, abelian magnetic solutions exist only for a specific range of threshold correction parameters. At small scales they correspond to the strong coupling region (Φ→∞) and the compactification region (φ→0). The non-perturbative potential V plays a crucial role at large scales, where it fixes the asymptotic values of Φ and φ to be at the minimum of V. (orig.)