2026-07-072026-07-07http://salesiana.dossiersoluciones.com/handle/123456789/55224The study of the special F^4 and R^4 in the effective action for the Spin(32)/Z_2 and type II strings sheds some light on D-brane calculus and on instanton contribution counting. The D-instanton case is discussed separately.latex 4 pages, crckapb style used. Cargese 1997 Gong show.v2 English corrected. v3 correction correctedHigh Energy Physics - Theorytext