CHARGED STRING SOLUTIONS WITH DILATON AND MODULUS FIELDS

We find charged, abelian, spherically symmetric solutions (in flat spa ce-time) corresponding to the effective action of D = 4 heterotic stri ng theory with the scale-dependent dilaton phi and modulus psi fields. We take into account perturbative (genus-one), moduli-dependent ''thr eshold'' corrections to the coupling function f(phi, phi) in the gauge field kinetic term f(phi, phi)F(munu)2, as well as the non-perturbati ve scalar potential V(phi, phi), 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) h ave the small scale region as the weak coupling region (phi --> -infin ity) with the modulus phi slowly varying towards smaller values. Stabl e, finite-energy, abelian magnetic solutions exist only for a specific range of threshold correction parameters. At small scales they corres pond to the strong coupling region (phi --> infinity) and the compacti fication region (phi --> 0). The non-perturbative potential V plays a crucial role at large scales, where it fixes the asymptotic values of phi and psi to be at the minimum of V.