F-THEORY ORIENTIFOLDS, M-THEORY ORIENTIFOLDS, AND TWISTED STRINGS

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