A note on toroidal compactifications of the type I superstring and other superstring vacuum configurations with 16 superchanges Original Research Article

We show that various disconnected components of the moduli space of superstring vacua with 16 superchanges admit a rationale in terms of BPS unorientifolds, 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.