Orientifold and Type II Dual Pairs
  In this paper we present a symmetry of a toroidally compactified type II string theory. This symmetry has the interpretaion that it interchanges the left and the right-moving worldsheet coordinates and reverses the orientations of some of the spatial coordinates. We also identify another discrete symmetry of the type II theory which is related to the above one by a nontrivial U-duality element of string theory. This symmetry, however, has trivial action on the worldsheet coordinates and corresponds to an improper T-duality rotation. We then construct examples of type II dual pairs in four dimensions by modding out the known type II dual pairs by the above symmetries. We show the explicit matching of the spectrum and supersymmetries in these examples. 
Alok Kumar