2026-07-072026-07-07http://salesiana.dossiersoluciones.com/handle/123456789/51718These notes are based on lectures presented at the 2001 Les Houches Summerschool ``Unity from Duality: Gravity, Gauge Theory and Strings''Lecture notes 2001 Les Houches Summerschool, LaTex, 147 pages. To be published in the proceedingsHigh Energy Physics - Theorytext