A Note on Toroidal Compactifications of the Type I Superstring and Other Superstring Vacuum Configurations with 16 Supercharges

We show that various disconnected components of the moduli space of superstring vacua with 16 supercharges admit a rationale in terms of BPS un-orientifolds, i.e. type I toroidal compactifications with constant non-vanishing but quantized vacuum expectation values of the NS×NS antisymmetric tensor. These include various heterotic vacua with reduced rank, known as CHL strings, and their dual type II (2,2) superstrings in D ≤ 6. Type I vacua without open strings allow for an interpretation of several disconnected components with NV = 10 − D. An adiabatic argument relates these unconventional type I superstrings to type II (4,0) superstrings without D-branes. The latter are connected by U-duality in D ≤ 6 to type II (2,2) superstrings. We also comment on the relation between some of these vacua and compactifications of the putative M-theory on unorientable manifolds as well as F-theory vacua.