CHARGED BLACK-HOLES IN EFFECTIVE STRING THEORY

We investigate the qualitative new features of charged dilatonic black holes which emerge when both the Yang-Mills and Gauss-Bonnet curvature corrections are included in the effective action. We consider these perturbative effects by an expansion up to second order in the inverse string tension on the four-dimensional Schwarzschild background and determine the back reaction. We calculate the thermodynamical functions and show that they can behave like those of the Garfinkle-Horowitz-Strominger solution or in a more conventional way, depending on the value of the magnetic charge q. Moreover, we find that for magnetic charge above a critical value, the temperature of the black hole has a maximum and goes to zero for a finite value of the mass. This indicates that the conventional Hawking evaporation law is modified by string theory at a classical level.