The problem of singularities of higher order curvature corrections in four dimensional string gravity

The influence of higher order - stringly inspired - curvature corrections to the classical General Relativity spherically symmetric solution is studied. These curvature corrections provide a singular contribution to the field equations because they generate higher derivatives of metric functions multiplied by a small parameter (the string coupling constant). Analytically and numerically, it is shown that in both Bosonic and Heterotic string gravity, the Schwarzschild solution is not recovered when the string coupling constant vanishes. It seems that only Superstring II low energy model does not induce any singular contribution in spherically symmetric static space times