Large N limit of extremal non-supersymmetric black holes

The large N limit of extremal non-supersymmetric Type-I five-dimensional st ring black holes is studied from the point of view of D-branes, We find tha t the agreement between the D-brane and the black-hole picture is due to an asymptotic restoration of supersymmetry in the large N limit in which both pictures are compared. In that limit Type-I string perturbation theory is effectively embedded into a Type-IIB perturbation theory with unbroken supe rsymmetric charges whose presence guarantees the non-renormalization of mas s and entropy as the effective couplings are increased. In this vein, we al so study the near-horizon geometry of the Type-I black hole using D5-brane probes to find that the low-energy effective action for the probe is identi cal to the corresponding one in the auxiliary Type-IIB theory in the large N limit. (C) 1999 Elsevier Science B.V.