RINGO2 data of QSO B0957+561. The Stokes parameters (q, u) of A and B at the three observing epochs are corrected for instrumental polarisation and depolarisation effects (see main text). We also display the means and root-mean-square deviations of both parameters for each quasar image (crosses and ellipses), as well as dashed open circles representing three different polarisation degrees: 1%, 2% and 3%.