Type I Superstrings without D-branes

Abstract

Notwithstanding the central role of D-branes in many recently proposed string dualities, several interesting type I vacua have been found without resorting directly to D-brane technology. In this talk, we analyze a three-generation SO(8)xU(12) chiral type I model with N=1 supersymmetry in D=4. It descends from the type IIB compactification on the Z orbifold and requires only Neumann boundary conditions, i.e. only the ubiquitous D9-branes (pan-branes). We also discuss a large class of 6D type I vacua that display rich patterns of Chan-Paton symmetry breaking/enhancement and various numbers of tensor multiplets. Finally, we briefly address issues raised by the conjectured heterotic - type I duality and by the relation between type I vacua and compactifications of the putative F-theory.