Twelve-Dimensional Aspects of Four-Dimensional N=1 Type I Vacua

Four-dimensional supergravity theories are reinterpreted in a 12-dimensional F-theory framework. The O(8) symmetry of N=8 supergravity is related to a reduction of F-theory on T_8, with the seventy scalars formally associated, by O(8) triality, to a fully compactified four-form A_4. For the N=1 type I model recently obtained from the type IIB string on the Z orbifold, we identify the K\"ahler manifold of the untwisted scalars in the unoriented closed sector with the generalized Siegel upper-half plane Sp(8,R)/(SU(4) \times U(1)). The SU(4) factor reflects the holonomy group of Calabi-Yau fourfolds.