NS Branes in Type I Theory

We consider novel nonperturbative effects of type I theories compactified on singular ALE spaces obtained by adding NS branes. Such effects include a description of small $E_8$ instantons at singularities.Comment: 6 pages, Harvmac.tex, updated referenc

Calculation of Nonperturbative Terms in Open String Models

Nonperturbative corrections in type II string theory corresponding to Riemann surfaces with one boundary are calculated in several noncompact geometries of desingularized orbifolds. One of these models has a complicated phase structure which is explored. A general condition for integrality of the numerical invariants is discussed

Consistency Conditions for Branes at Orbifold Singularities

We discuss consistency conditions for branes at orbifold singularities. The conditions have a world-sheet interpretation in terms of tadpole cancellation and a space-time interpretation in terms of anomalies. As examples, we consider type II and type I branes on $C^2/Z_M$ orbifolds. We give orientifold constructions of phases of type I or heterotic string theory, involving branches with extra tensor multiplets, which arise when small SO(32) instantons sit on orbifold singularities.Comment: harvmac, 14 pages, added reference

H-Dyons and S-Duality

We present a relatively simple argument showing that the H-dyon states required by S-duality of the heterotic string on $T^6$ are present provided that the BPS dyons required by S-duality of N=4 supersymmetric Yang-Mills theory are present. We also conjecture and provide evidence that H-dyons at singularities where the nonperturbative gauge symmetry is completely broken are actually BPS dyons.Comment: 16 pages, uses harvmac.tex, arguments are drastically changed but the conclusions remain unchange

New Phases of String Theory and 6d RG Fixed Points via Branes at Orbifold Singularities

We discuss type II and type I branes at general ADE type orbifold singularities. We show that there are new phases of type I or heterotic string theory in six dimensions, involving extra tensor multiplets, which arise when small instantons sit on orbifold singularities. The theories with extra tensor multiplets are explicitly constructed via orientifolds. The world-volume theories in type IIB or type I five-branes at orbifold singularities lead to the existence of several infinite classes of six dimensional, interacting, renormalization group fixed point theories.Comment: Harvmac, 28 pages, small typo correcte

Duality without Supersymmetry: The Case of the SO(16)xSO(16) String

We extend strong/weak coupling duality to string theories without spacetime supersymmetry, and focus on the case of the unique ten-dimensional, nonsupersymmetric, tachyon-free $SO(16)\times SO(16)$ heterotic string. We construct a tachyon-free heterotic string model that interpolates smoothly between this string and the ten-dimensional supersymmetric $SO(32)$ heterotic string, and we construct a dual for this interpolating model. We find that the perturbative massless states of our dual theories precisely match within a certain range of the interpolation. Further evidence for this proposed duality comes from a calculation of the one-loop cosmological constant in both theories, as well as the presence of a soliton in the dual theory. This is therefore the first known duality relation between nonsupersymmetric tachyon-free string theories. Using this duality, we then investigate the perturbative and nonperturbative stability of the $SO(16)\times SO(16)$ string, and present a conjecture concerning its ultimate fate.Comment: 15 pages, LaTeX, 3 figure

Triple Intersections and Geometric Transitions

The local neighborhood of a triple intersection of fivebranes in type IIA string theory is shown to be equivalent to type IIB string theory on a noncompact Calabi-Yau fourfold. The phases and the effective theory of the intersection are analyzed in detail.Comment: earlier reference adde