I'm wondering if the norwegian play they were putting on stage ,"La disparu" I believe it was calles, did really exist or was it invented for the film because I know that The Magic Mountain, the next play Lucas wanted to make, is real.
"La disparue" was invented for the film, as was mentioned in the interviews on the Criterion DVD extras. There is no Norwegian playwright named Karen Bergen, the supposed author of the play. I got the impression it was meant to be vaguely in the genre of Ibsen. I don't think there is a play of The Magic Mountain. What Lucas was proposing was to dramatize Thomas Mann's novel, The Magic Mountain. The German title, Die Zauberberg, was Lucas had written on the poster which we saw on the wall in the cellar and on which he was diagramming the play. However, he could never have put that play on while the Germans were in Paris. It would have been instantly banned, since Mann, was a prominent anti-fascist who had fled Germany and was living in the US. It couldn't have been done until after the Liberation. And, indeed, the play that Marion and Bernard were doing in the epilogue could have been The Magic Mountain -- much of which takes place in a TB sanatorium.