  We propose an analogue of spin fields for the relativistic RNS-particle in 4
dimensions, in order to describe Ramond-Ramond states as "two-particle"
excitations on the world line. On a natural representation space we identify a
differential whose cohomology agrees with RR-fields equations. We then discuss
the non-linear theory encoded in deformations of the latter by background
fields. We also formulate a sigma model for this spin field from which we
recover the RNS-formulation by imposing suitable constraints.
