Comments on Gepner Models and Type I Vacua in String Theory
  We construct open descendants of Gepner models, concentrating mainly on the six-dimensional case, where they give type I vacua with rich patterns of Chan-Paton symmetry breaking and various numbers of tensor multiplets, including zero. We also relate the models in $D < 10$ without open sectors, recently found by other authors, to the generalized Klein-bottle projections allowed by the crosscap constraint. 
C. Angelantonj (U. Aquila), M. Bianchi, G. Pradisi, A. Sagnotti and   Ya.S. Stanev (U. Roma "Tor Vergata")