R-band relative Stokes parameters of VI Cyg 12 (black circles) in comparison with two other standards: BD+32.3739 (blue triangles) and HD 212311 (green squares). The horizontal red lines and the pink areas represent Q/I and U/I values for VI Cyg 12 from Schmidt et al. (1992) with corresponding uncertainties. For ease of visualization, the values for the two other stars are shifted so that their average Stokes parameters match the red line. In spite of larger photon noise uncertainties, it is clear that the Stokes parameters of VI Cyg 12 are significantly variable and systematically deviate from their catalog values during long periods of time.