The R ξ gauges are a generalization of the Lorenz gauge applicable to theories expressed in terms of an action principle with Lagrangian density L . Instead of fixing the gauge by constraining the gauge field a priori, via an auxiliary equation, one adds a gauge breaking term to the "physical" (gauge invariant) Lagrangian δ δ L = − − ( ∂ ∂ μ μ A μ μ ) 2 2 ξ ξ