Crosscaps, boundaries and T-duality

Open descendants with boundaries and crosscaps of non-trivial automorphism type are studied. We focus on the case where the bulk symmetry is broken to a Z(2) orbifold subalgebra. By requiring positivity and integrality for th e open sector, we derive a unique crosscap of automorphism type g is an ele ment of Z(2) and a corresponding g-twisted Klein bottle for a charge conjug ation invariant. As a specific example, we use T-duality to construct the d escendants of the true diagonal invariant with symmetry preserving crosscap s and boundaries. (C) 2000 Elsevier Science B.V. All rights reserved.