Macroscopic and microscopic description of black diholes

We study configurations consisting of a pair of non-extremal black holes in four dimensions, both with the same mass, and with charges of the same mag nitude but opposite sign - diholes, for short. We present such exact soluti ons for Einstein-Maxwell theory with arbitrary dilaton coupling, and also s olutions to the U(1)(4) theories that arise from compactified string/M-theo ry. Despite the fact that the solutions are very complicated, physical prop erties of these black holes, such as their area, charge, and interaction en ergy, admit simple expressions. We also succeed in providing a microscopic description of the entropy of these black holes using the 'effective string ' model, and taking into account the interaction between the effective stri ng and antistring. (C) 2001 Published by Elsevier Science B.V.