  We write the type IIB worldsheet action in classes of bosonic curved
backgrounds threaded with Ramond-Ramond fluxes. The fixing of the kappa
symmetry in the light-cone gauge and the use of the Bianchi identities of the
supergravity theory lead to an expression of a relatively simple form, yet rich
with new physical information about how fundamental strings react to the
presence of RR fields. The results are useful in particular to the study of
vacuum structure and dynamics in the context of the Holographic duality; and to
possibly formulate an open-closed string duality at the level of the
worldsheet.
