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 commute with the operations of picture changing, BRST cohomologies at different pictures need not agree. We show explicitly that RamondRamond (RR) zero-momentum physical states are inequivalent at different pictures, and prove that non-zero momentum physical states are equivalent in all pictures. We find that D-brane states represent BRST classes that are nonpolynomial on the superghost zero modes, while RR gauge fields appear as polynomial BRST classes. We also prove that in x-cohomology, the cohomology where the zero mode of the spatial coordinates is included, there is a unique ghost-number one BRST class responsible for the Green-Schwarz anomaly, and a unique ghost number minus one BRST class associated with RR charge. ⋆ E-mail address: [email protected]. Supported in part by CNPq fellowship 300256/94-9. † E-mail address: [email protected]. Supported in part by D.O.E. contract DE-FC02-94ER40818, and a fellowship of the John Simon Guggenheim Memorial Foundation.