arXiv:hep-th/0307183v2 5 Aug 2003 CAB-IB/2902603 hep-th/0307183 Type IIB orientifolds on Gepner points Gerardo Aldazabal 1 , Eduardo C. Andr´ es 1 , Mauricio Leston 2 and Carmen N´ u˜ nez 2 1 Instituto Balseiro, CNEA, Centro At´ omico Bariloche, 8400 S.C. de Bariloche, and CONICET, Argentina. 2 Instituto de Astronom´ ıa y F´ ısica del Espacio, CONICET C.C. 67 - Suc. 28, 1428 Buenos Aires, Argentina, and Departamento de F´ ısica, FCEN-UBA, Argentina Abstract We study various aspects of orientifold projections of Type IIB closed string theory on Gepner points in different dimensions. The open string sector is introduced, in the usual constructive way, in order to cancel RR charges car- ried by orientifold planes. Moddings by cyclic permutations of the internal N = 2 superconformal blocks as well as by discrete phase symmetries are im- plemented. Reduction in the number of generations, breaking or enhancements of gauge symmetries and topology changes are shown to be induced by such moddings. Antibranes sector is also considered; in particular we show how non supersymmetric models with antibranes and free of closed and open tachyons do appear in this context. A systematic study of consistent models in D =8 dimensions and some illustrative examples in D = 6 and D = 4 dimensions are presented.