  In a recent paper it is claimed that vacuum birefringence has been
experimentally observed for the first time by measuring the degree of
polarization of visible light from a Magnetar candidate, a neutron star with a
magnetic field presumably as large as $B\sim 10^{13}$~G. The role of such a
strong magnetic field is twofold. First, the surface of the star emits, at each
point, polarized light with linear polarization correlated with the orientation
of the magnetic field. Depending on the relative orientation of the magnetic
axis of the star with the direction to the distant observer, a certain degree
of polarization should be visible. Second, the strong magnetic field in the
vacuum surrounding the star could enhance the effective degree of polarization
observed: vacuum birefringence. We compare experimental data and theoretical
expectations concluding that the conditions to support a claim of strong
evidence of vacuum birefringence effects are not met.
