Non-supersymmetric type I strings with zero vacuum energy

We study open descendants of non-supersymmetric type IIB asymmetric (freely acting) orbifolds with zero cosmological constant. A generic feature of th ese models is that supersymmetry remains unbroken on the brane at all mass levels, while it is broken in the bulk in a way that preserves Fermi-Bose d egeneracy in both the massless and massive (closed string) spectrum. This p roperty remains valid in the heterotic dual of the type II model but only f or the massless excitations. A possible application of these constructions concerns scenarios of low-energy supersymmetry breaking with large dimensio ns. (C) 1999 Elsevier Science B.V. All rights reserved.