1995A&A...296..390K

We present the first detection of linear polarization of the UV/optical counterpart of the low mass X-ray binary Cyg X-2. The measurements of Cyg X-2, and of 10 nearby field stars were performed using the Bernard Lyot Telescope at Pic du Midi Observatory between July 20 and 26, 1993. The degree of linear polarization in the direction of Cyg X-2 was measured in the U+B+V range at orbital phases 0.4, 0.5, 0.6 and 0.8 and found to be constant with a mean value of 0.29±.035% and a position angle of 36±3deg. Although we can only set an upper limit of about 0.25% and 0.35% respectively, to the amplitude of the variation of the two components of the polarization, we deduce that Cyg X-2 may be intrinsically polarized, by comparing its position in the Stokes plane with other nearby field stars we observed with known distances.