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We study nonlinear vacuum electrodynamics in the first-order formulationproposed by Pleba\'nski. We analyze in detail the equations of motion, andidentify conditions for which a singularity can occur for the time derivativeof one of the field components. The resulting degenerate behavior can give riseto a shock wave with a reduction of the local number of degrees of freedom. Weuse an example model to illustrate the occurrence of superluminal propagationfor field values approaching the singularity.

Degenerate behavior in nonlinear vacuum electrodynamics