  We give an example of topological theory whose Hilbert space contains
physical objects: the N=2 supersymmetric Lagrangian of spin-one particles
moving in D-dimensional space-time equals the Lagrangian of a topological sigma
model in a (D+2)-dimensional target. The equivalence is valid for a curved
space-time. As an application, we calculate the deviation of spin-one particles
in a Schwarzschild background (gravitational Stern-Gerlach effect).
