  The Chern-Simons modification to general relativity in four dimensions
consists of adding to the Einstein-Hilbert term a scalar field that couples to
the first class Pontryagin density. In this theory, which has attracted
considerable attention recently, the Schwarzschild metric persists as an exact
solution, and this is why this model resists several observational constraints.
In contrast, the spinning black hole solution of the theory is not given by the
Kerr metric but by a modification of it, so far only known for slow rotation
and small coupling constant. In the present paper, we show that, in this
approximation, the null geodesic equation can be integrated, and this allows us
to investigate the shadow cast by a black hole. We discuss how, in addition to
the angular momentum of the solution, the coupling to the Chern-Simons term
deforms the shape of the shadow.
