The supersymmetric action of a type IIA D = 10 superstring in a N = 2a
, D = 10 supergravity background can be derived by double-dimensional
reduction of the action of a supermembrane coupled to D = 11 supergrav
ity. We demonstrate that the background Ramond-Ramond fields appear in
the resulting superstring action with an extra exponential factor of
the dilaton.