Etherington's Distance Duality with Birefringence
  We consider light propagation in a spacetime whose kinematics allow weak
birefringence, and whose dynamics have recently been derived by gravitational
closure. Revisiting the definitions of luminosity and angular diameter
distances in this setting, we present a modification of the Etherington
distance duality relation in a weak gravitational field around a point mass.
This provides the first concrete example of how the non-metricities implied by
gravitational closure of birefringent electrodynamics affect observationally
testable relations.
