EXTREMALITY VERSUS SUPERSYMMETRY IN STRINGY BLACK-HOLES

We study general black-hole solutions of the low-energy string effecti ve action in arbitrary dimensions using a general metric that can desc ribe them all in a unified way both in the extreme and non-extreme cas es. We calculate the mass, temperature and entropy and study which rel ations amongst the charges and the mass lead to extremality. We find t hat the temperature always vanishes in the extreme limit and we find t hat, for a set of n charges (no further reducible by duality) there ar e 2((n-1)) combinations of the charges that imply extremality. Not all of these combinations can be central charge eigenvalues and, thus, th ere are in general extreme black holes which are not supersymmetric (o r ''BPS-saturated''). In the N = 8 supergravity case we argue that the existence of roughly as many supersymmetric and non-supersymmetric ex treme black holes suggests the existence of an underlying twelve-dimen sional structure. (C) 1998 Published by Elsevier Science B.V.