EXTREMAL BLACK-HOLES AS FUNDAMENTAL STRINGS

We show that polarization-dependent string-string scattering provides new evidence for the identification of the Dabholkar-Harvey (DH) strin g solution with the heterotic string itself. First, we construct excit ed versions of the DH solution which carry arbitrary left-moving waves yet are annihilated by half the supersymmetries. These solutions corr espond in a natural way to Bogomolny-bound-saturating excitations of t he ground state of the heterotic string. When the excited string solut ions are compactified to four dimensions, they reduce to Sen's family of extremal black hole solutions of the toroidally compactified hetero tic string. We then study the large impact parameter scattering of two such string solutions. We develop methods which go beyond the metric on moduli space approximation and allow us to read off the subleading polarization-dependent scattering amplitudes. We find perfect agreemen t with heterotic string tree amplitude predictions for the scattering of corresponding string states. Taken together, these results clearly identify the string states responsible for Sen's extremal black hole e ntropy. We end with a brief discussion of implications for the black h ole information problem.