Orientifolds, Mirror Symmetry and Superpotentials
  We consider orientifolds of Calabi-Yau 3-folds in the context of Type IIA and Type IIB superstrings. We show how mirror symmetry can be used to sum up worldsheet instanton contributions to the superpotential for Type IIA superstrings. The relevant worldsheets have the topology of the disc and ${\bf RP^2}$. 
Bobby Acharya, Mina Aganagic, Kentaro Hori and Cumrun Vafa