F-theory orientifolds, M-theory orientifolds, and twisted strings

Orientifolds of the type IIB superstring that descend from F-theory and M-theory orbifolds are studied perturbatively. One finds strong evidence that a previously ignored twisted open string is required in these models. An attempt is made to interpret the J-type torsion in F-theory where one finds a realization of Gimon and Johnson's models which does not require these twisted strings. S-duality also provides evidence for these strings.