  We study the set of trapped photons of a subcritical (a<M) Kerr spacetime as
a subset of the phase space. First, we present an explicit proof that the
photons of constant Boyer--Lindquist coordinate radius are the only photons in
the Kerr exterior region that are trapped in the sense that they stay away both
from the horizon and from spacelike infinity.
  We then proceed to identify the set of trapped photons as a subset of the
(co-)tangent bundle of the subcritical Kerr spacetime. We give a new proof
showing that this set is a smooth 5-dimensional submanifold of the (co-)tangent
bundle with topology $SO(3)\times\mathbb R^2$ using results about the
classification of 3-manifolds and of Seifert fiber spaces.
  Both results are covered by the rigorous analysis of Dyatlov [5]; however,
the methods we use are very different and shed new light on the results and
possible applications.
