Dilatonic black holes in higher curvature string gravity. II. Linear stability

We give analytical arguments and demonstrate numerically the existence of black hole solutions of the 4D effective superstring action in the presence of Gauss-Bonnet quadratic curvature terms. The solutions possess nontrivial dilaton hair. The hair, however, is of "secondary type," in the sense that the dilaton charge is expressed in terms of the black hole mass. Our solutions are not covered by the assumptions of existing proofs of the "no-hair" theorem. We also find some alternative solutions with singular metric behavior, but finite energy. The absence of naked singularities in this system is pointed out.