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 {\bf Z}_2 \otimes {\bf Z}_2^{\prime} 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 strong-weak 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. 
Gary Shiu and S.-H. Henry Tye