  We investigate photon surfaces and their stability in a less symmetric
spacetime, a general static warped product with a warping function acting on a
Riemannian submanifold of codimension two. We find a one-dimensional
pseudopotential that gives photon surfaces as its extrema regardless of the
spatial symmetry of the submanifold. The maxima and minima correspond to
unstable and stable photon surfaces, respectively. It is analogous to the
potential giving null circular orbits in a spherically symmetric spacetime. We
also see that photon surfaces indeed exist for the spacetimes which are
solutions to the Einstein equation. The parameter values for which the photon
surfaces exist are specified. As we show finally, the pseudopotential arises
due to the separability of the null geodesic equation, and the separability
comes from the existence of a Killing tensor in the spacetime. The result leads
to the conclusion that photon surfaces may exist even in a less symmetric
spacetime if the spacetime admits a Killing tensor.
