BRST-invariant transitions between closed and open strings

We consider the problem of how closed strings couple to open strings. We obtain a Becchi-Rouet-Stora-Tyutin- (BRST-) invariant first-quantized operator which codes off-shell transitions between a single closed-string state and a single open-string state is obtained in both the bosonic and the fermionic language for the ghosts. We confirm that implies the correct scattering amplitudes for one closed string in its tachyonic and massless states and any number of tachyon open strings. Our BRST-invariant transition operator is then used in a novel analysis of the Cremmer-Scherk-Higgs mechanism, in which the open-string photon and the closed-string antisymmetric tensor get a mass. We close with some remarks on the possible usefulness of our methods for string field theory.