Notes on Orientifolds of Rational Conformal Field Theories
  We review and develop the construction of crosscap states associated with parity symmetries in rational conformal field theories. A general method to construct crosscap states in abelian orbifold models is presented. It is then applied to rational U(1) and parafermion systems, where in addition we study the geometrical interpretation of the corresponding parities. 
Ilka Brunner and Kentaro Hori