Bose-Fermi degeneracy and duality in non-supersymmetric strings

Following Kachru, Kumar and Silverstein, we construct a set of non-supersym metric Type II string models which have equal numbers of bosons and fermion s at each mass level. The models are asymmetric Z(2) x Z(2)' orbifolds. We demonstrate that this Bose-Fermi degeneracy feature implies that both the o ne-loop and the two-loop contributions to the cosmological constant vanish. We conjecture that the cosmological constant actually vanishes to all loop s. We construct a strong-weak dual pair of models, both of which have Bose- Fermi degeneracy. This implies that at least some of the non-perturbative c orrections to the cosmological constant are absent. (C) 1999 Elsevier Scien ce B.V.