BPS States in N=3 Superstrings

The N = 3 string models are special solutions of the type II perturbative string theories. We present explicit expressions for the helicity supertraces,which count the number of the perturbative BPS multiplets. Assuming the non-perturbative duality $(S\leftrightarrow T)$ of the heterotic string on T^6 and type II on K_3 x T^2 valid in N = 4 theories, we derive the N = 3 non-perturbative BPS mass formula by ``switching off" some of the N = 4 charges and ``fixing" to special values some of the N = 4 moduli. This operation corresponds to a well-defined Z_2 projection acting freely on the compactification manifold. The consistency of this projection and the precise connection of the N = 4 and N = 3 BPS spectrum is shown explicitly in several type II string constructions. The heterotic N = 3 and some asymmetric type II constructions turn out to be non-perturbative with the S moduli fixed at the self-dual point S=i. Some of the non-perturbative N = 3 type II are defined in the context of F-theory. The bosonic sector of the N=3 string effective action is also presented. This part can be useful for the study of 4d black holes in connection with the asymptotic density of BPS states in N = 3 string theory.