Orientifolds of non-supersymmetric, asymmetric orbifolds Original Research Article

We consider certain four-dimensional supersymmetric and non-supersymmetric asymmetric orbifolds with vanishing cosmological constant up to two loops and gauge the world-sheet parity transformation. This leads to new string vacua, in which Dp- and D(p - 4)-branes or Dp and D(p-4) are identified. Moreover, it is shown that different degrees of supersymmetry can be realized in the bulk and on the brane. We show that for non-supersymmetric models the cosmological constant still vanishes at one-loop order.