2026-07-072026-07-07http://salesiana.dossiersoluciones.com/handle/123456789/55311The boundary conditions of a non-trivial string background are classified. To this end we need traces on various spaces of conformal blocks, for which generalizations of the Verlinde formula are presented.7 pages, LaTeX. Slightly extended version of a talk given by C. Schweigert at the 31st International Symposium Ahrenshoop on the Theory of Elementary Particles, Buckow, Germany, September 1997High Energy Physics - Theorytext