S-DUALITY AND THE ENTROPY OF BLACK-HOLES IN HETEROTIC STRING THEORY

Four-dimensional heterotic string effective action is known to admit n on-rotating electrically and magnetically charged black hole solutions . The partition function and entropy is computed for electrically char ged black holes and is vanishing in some extremal limit. For the magne tically charged black holes the entropy is also argued to be vanishing in the same extremal limit when these black hole solutions are relate d by S-duality transformations.