  Under the assumption of a specific nonminimal coupling of torsion to Under
the assumption of a specific nonminimal coupling of torsion to
electromagnetism, spacetime is birefringent in the presence of a gravitational
field leading to depolarization of light emitted from extended astrophysical
sources. We use polarimetric data of the magnetic white dwarf RE J0317-853 to
set for the very first time constraints on the essential coupling constant for
this effect, giving k^2 <22 m^2. electromagnetism, spacetime is birefringent in
the presence of a gravitational field leading to depolarization of light
emitted from extended astrophysical sources. We use polarimetric data of the
magnetic white dwarf RE J0317-853 to set for the very first time constraints on
the essential coupling constant for this effect, giving k^2 <22 m^2.
