Tensors fromK3orientifolds

Abstract

Recently Gimon and Johnson and Dabholkar and Park have constructed type I theories on $K3\mathrm{}$ orbifolds. The spectra differ from that of type I on a smooth $K3\mathrm{}$, having extra tensors. We show that the orbifold theories cannot be blown up to smooth $K3\mathrm{}$’s, but rather ${\mathbf{Z}}_2$ orbifold singularities always remain. Douglas’s recent proposal to use $D$-branes as probes is useful in understanding the geometry. The ${\mathbf{Z}}_2$ singularities are of a new type, with a different orientifold projection from those previously considered. We also find a new world-sheet consistency condition that must be satisfied by orientifold models.