AN SL(2, Z) MULTIPLET OF BLACK-HOLES IN D=4 TYPE-II SUPERSTRING THEORY

It is well-known that the conjectured SL(2, Z) invariance of type IIB string theory in ten dimensions also persists in lower dimensions when the theory is compactified on tori. By making use of this recent obse rvation, we construct an infinite family of magnetically charged black hole solutions of type II superstring theory in four space-time dimen sions. These solutions are characterized by two relatively prime integ ers corresponding to the magnetic charges associated with the two gaug e fields (from NS-NS and R-R sectors) of the theory and form an SL(2, Z) multiplet. In the extremal limit these solutions are stable as they are prevented from decaying into black holes of lower masses by a 'ma ss gap' equation. (C) 1998 Published by Elsevier Science B.V.