Birkhoff's Theorem in Conformal Gravity

We prove Birkhoff's theorem in conformal gravity: The most general spherically symmetric, electrovac solution is static, and hence absolutely stable against spherically symmetric perturbations. This result goes beyond the zero-energy theorem since it applies even when the solution is not asymptotically flat. On the basis of the known examples, we conjecture that any field theory lacking linearized spin-0 modes possesses a Birkhoff theorem.