ON THE PICTURE DEPENDENCE OF RAMOND-RAMOND COHOMOLOGY

Closed string physical states are BRST cohomology classes computed on the space of states annihilated by b(0)(-). Since b(0)(-) does not com mute with the operations of picture changing, BRST cohomologies at dif ferent pictures need not agree. We show explicitly that Ramond-Ramond (RR) zero-momentum physical states are inequivalent at different pictu res, and prove that nonzero-momentum physical states are equivalent in all pictures. We find that D-brane states represent BRST classes that are non-polynomial on the superghost zero-modes, while RR gauge field s appear as polynomial BRST classes. We also prove that in x-cohomolog y, the cohomology where the zero-mode of the spatial coordinates is in cluded, there is a unique ghost-number one BRST class responsible for the Green-Schwarz anomaly, and a unique ghost number minus one BRST cl ass associated with RR charge. (C) 1998 Elsevier Science B,V.