Magnetic Fluxes, NS-NS BB Field and Shifts in Four-Dimensional Orientifolds

We discuss the interplay between freely acting orbifold actions, discrete deformations and internal uniform magnetic fields in four-dimensional orientifold models.Comment: 12 pages, 3 figures, LaTeX file. Contribution to the Proceedings of the "String Phenomenology 2003" conference, Durham, England, 29th July - 4th August 200

Type-I vacua from non-geometric Orbifolds

Some examples of Type-I vacua related to non geometric orbifolds are shown. In particular, the open descendants of the diagonal $Z_3$ orbifold are compared with the geometric ones. Although not chiral, these models exhibit some interesting properties, like twisted sectors in the open-string spectra and the presence of ``quantized'' geometric moduli, a key ingredient to ensure their perturbative consistency and to explain the rank reduction of their Chan-Paton groups.Comment: Contribution to the Proceedings of IX Marcel Grossman Meeting, Rome, ITALY, July 2-8, 200

Fixed point resolution in extended WZW-models

A formula is derived for the fixed point resolution matrices of simple current extended WZW-models and coset conformal field theories. Unlike the analogous matrices for unextended WZW-models, these matrices are in general not symmetric, and they may have field-dependent twists. They thus provide non-trivial realizations of the general conditions presented in earlier work with Fuchs and Schweigert.Comment: 21 pages, Phyzz

Non-geometric orbifolds and wrapping rules

We show that the number of half-supersymmetric p-branes in the Type II theories compactified on orbifolds is determined by the wrapping rules recently introduced, provided that one accounts correctly for both geometric and non-geometric T-dual configurations. Starting from the Type II theories compactified on K3, we analyze their toroidal dimensional reductions, showing how the resulting half-supersymmetric p-branes satisfy the wrapping rules only by taking into account all the possible higher-dimensional origins. We then consider Type II theories compactified on the orbifold T^6/(Z_2 \times Z_2 ), whose massless four-dimensional theory is an N=2 supergravity. Again, the wrapping rules are obeyed only if one includes the complete orbit of the T-duality group, namely either Type IIA or Type IIB theories compactified on either the geometric or the non-geometric T-dual orbifold. Finally, we comment on the interpretation of our results in the framework of the duality between the Heterotic string compactified on K3 \times T^2 and the Type II string compactified on a Calabi-Yau threefold.Comment: 26 pages, 8 table

A classifying algebra for boundary conditions

We introduce a finite-dimensional algebra that controls the possible boundary conditions of a conformal field theory. For theories that are obtained by modding out a Z_2 symmetry (corresponding to a so-called D_odd-type, or half-integer spin simple current, modular invariant), this classifying algebra contains the fusion algebra of the untwisted sector as a subalgebra. Proper treatment of fields in the twisted sector, so-called fixed points, leads to structures that are intriguingly close to the ones implied by modular invariance for conformal field theories on closed orientable surfaces.Comment: 12 pages, LaTe

Non-tachyonic open descendants of the 0B string theory

We use the crosscap constraint to construct open descendants of the 0B string compactified on $T^6 /Z_3$ and on $T^4/Z_2$ free of tachyons both in the closed and in the open unoriented sectors. In four dimensions the construction results in a Chan-Paton gauge group $U(8)\otimes U(12)\otimes U(12)$ with three generations of chiral fermions in the representations $(\overline{8},1,\overline{12}) +(8,12,1)+(1,\overline{66},1)+(1,1,66)$.Comment: 13 pages, Latex. Minor corrections. Final version to appear on Phys.Lett.

Open Strings and Electric Fields in Compact Spaces

We analyse open strings with background electric fields in the internal space, T-dual to branes moving with constant velocities in the internal space. We find that the direction of the electric fields inside a two torus, dual to the D brane velocities, has to be quantised such that the corresponding direction is compact. This implies that D-brane motion in the internal torus is periodic, with a periodicity that can be parametrically large in terms of the internal radii. By S-duality, this is mapped into an internal magnetic field in a three torus, a quantum mechanical analysis of which yields a similar result, i.e. the parallel direction to the magnetic field has to be compact. Furthermore, for the magnetic case, we find the Landau level degeneracy as being given by the greatest common divisor of the flux numbers. We carry on the string quantisation and derive the relevant partition functions for these models. Our analysis includes also the case of oblique electric fields which can arise when several stacks of branes are present. Compact dimensions and/or oblique sectors influence the energy loss of the system through pair-creation and thus can be relevant for inflationary scenarios with branes. Finally, we show that the compact energy loss is always larger than the non-compact one.Comment: 44 pages, 4 figures; v2 corrections in section 8, one reference adde