An orientifold of Type-IIB theory on K3

A new orientifold of Type-IIB theory on K3 is constructed that has N = 1 supersymmetry in six dimensions. The orientifold symmetry consists of a Z(2) involution of K3 combined with orientation reversal on the world-sheet. The closed-string sector in the resulting theory contains nine tensor multiplets and twelve neutral hypermultiplets in addition to the gravity multiplet, and is anomaly free by itself. The open-string sector contains only five-branes and gives rise to maximal gauge groups SO(16) or U(8) x U(8) at different points in the moduli space. Anomalies are canceled by a generalization of the Green-Schwarz mechanism that involves more than one tensor multiplets.