Bose-Fermi degeneracy and duality in non-supersymmetric strings

Following Kachru, Kumar and Silverstein, we construct a set of non-supersymmetric Type II string models which have equal numbers of bosons and fermions at each mass level. The models are asymmetric Z2⊗Z2′ orbifolds. We demonstrate that this Bose-Fermi degeneracy feature implies that both the one-loop and the two-loop contributions to the cosmological constant vanish. We conjecture that the cosmological constant actually vanishes to all loops. We construct a strongweak dual pair of models, both of which have Bose-Fermi degeneracy. This implies that at least some of the non-perturbative corrections to the cosmological constant are absent.