Fluxes and branes in type II vacua and M-theory geometry with G2 and spin (7) holonomy

We discuss fluxes of RR and NSNS background fields in type II string compactifications on non-compact Calabi-Yau threefolds together with their dual brane description which involves bound states of branes. Simultaneously turning on RR and NSNS 2-form fluxes in an 1/2 supersymmetric way can be geometrically described in M-theory by a SL(2, Z) family of metrics of G(2) holonomy, On the other hand, if the flux configuration only preserves 1/4 of supersymmetries. we postulate the existence of a new eight-dimensional manifold with spin(7) holonomy, which does not seem to fit into the classes of known examples. The latter situation is dual to a 1/4 supersymmetric web of branes on the deformed conifold. In addition to the 2-form fluxes, we also present some considerations on type IIA NSNS 4-form and 6-form fluxes. (C) 2002 Elsevier Science B.V. All rights reserved.