  Tree-level scattering amplitudes for a scalar particle coupled to an
arbitrary number N of photons and a single graviton are computed. We employ the
worldline formalism as the main tool to compute the irreducible part of the
amplitude, where all the photons and the graviton are directly attached to the
scalar line, then derive a tree replacement rule to construct the reducible
parts of the amplitude which involve irreducible pure N-photon two-scalar
amplitudes where one photon line emits the graviton. We test our construction
by verifying the on-shell gauge and diffeomorphism Ward identities, at
arbitrary N.
