  The effects of light propagation in constant magnetic and electric
backgrounds are considered in the framework of the effective action approach.
We use the exact analytic series representation for the one-loop effective
action of QED and apply it to the birefringence effect. Analytical results for
the light velocity modes are obtained for weak and strong field regime. We
present asymptotic formulae for the light velocity modes for the ultra strong
magnetic field which can be realized in some neutron stars.
