  We investigate the Pleba\'nski class of electrodynamical theories, i.e.,
theories of nonlinear vacuum electrodynamics that derive from a
Lorentz-invariant Lagrangian (or Hamiltonian). In any such theory the light
rays are the lightlike geodesics of two optical metrics that depend on the
electromagnetic background field. A set of necessary and sufficient conditions
is found whose fulfillment secures that the optical metrics are causal in the
sense that the light rays are lightlike or timelike with respect to the
underlying space-time metric. Thereupon we derive conditions on the Lagrangian,
or the Hamiltonian, of the theory such that the causality conditions are
satisfied for all background fields. (The allowed values of the field strength
tensor are those for which the excitation tensor is finite and real.) The
general results are illustrated with several examples.
