Comments on the classification of orientifolds

The simple current construction of orientifolds based on rational conformal field theories is reviewed. When applied to SO(16) level 1, one can describe all ten-dimensional orientifolds in a unified framework.Comment: 9 pages, Contribution to proceedings of RTN-workshop in Leuven, Belgium, September 200

Theory and Phenomenology of Type I strings and M-theory

The physical motivations and the basic construction rules for Type I strings and M-theory compactifications are reviewed in light of the recent developments. The first part contains the basic theoretical ingredients needed for building four-dimensional supersymmetric models, models with broken supersymmetry and for computing low-energy actions and quantum corrections to them. The second part contains some phenomenological applications to brane world scenarios with low values of the string scale and large extra dimensions.Comment: 129 pages, 7 eps figures, LaTeX, version to appear in Class. Quantum Gra

Crosscaps, Boundaries and T-duality

Open descendants with boundaries and crosscaps of non-trivial automorphism type are studied. We focus on the case where the bulk symmetry is broken to a Z 2 orbifold subalgebra. By requiring positivity and integrality for the open sector, we derive a unique crosscap of automorphism type g 2 Z 2 and a corresponding g-twisted Klein bottle for a charge conjugation invariant. As a specic example, we use T-duality to construct the descendants of the true diagonal invariant with symmetry preserving crosscaps and boundaries. 1 Introduction The general prescription to construct open unoriented strings from closed oriented ones is known as the method of open descendants [1]. It is not limited to circle compactications and orbifolds of circles, as orientifolds are. In short, one has to nd a set of crosscap and boundary coecients from which one can calculate the Klein bottle, annulus and M\u7fobius strip partition function that together with the torus generate the full spectrum of the ope..

Open descendants of non-diagonal invariants

The open descendants of simple current automorphism invariants are constructed. We consider the case where the order of the current is two or odd. We prove that our solutions satisfy the completeness conditions, positivity and integrality of the open and closed sectors and the Klein bottle constraint (apart from an interesting exception). In order to do this, we derive some new relations between the tensor Y and the fixed point conformal field theory. Some non-standard Klein After a ten year period of neglect, open strings have received more interest recently for a variety of reasons: their rôle in the duality picture, the discovery of D-branes and the appearance of non-commutative geometry, and recent developments in phenomenological string theory, such as brane worl