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The coupling of the electromagnetic field directly with gravitational gaugefields leads to new physical effects that can be tested using astronomicaldata. Here we consider a particular case for closer scrutiny, a specificnonminimal coupling of torsion to electromagnetism, which enters into ametric-affine geometry of space-time. We show that under the assumption of thisnonminimal coupling, spacetime is birefringent in the presence of such agravitational field. This leads to the depolarization of light emitted fromextended astrophysical sources. We use polarimetric data of the magnetic whitedwarf ${RE J0317-853}$ to set strong constraints on the essential couplingconstant for this effect, giving $k^2 \lsim (19 {m})^2 $.Comment: Statements about Moffat's NGT modified. Accepted for publication in  Phys.Rev.

An astronomical search for evidence of new physics: Limits on gravity-induced birefringence from the magnetic white dwarf RE J0317-853