Relation between δA_U and δA_V after ew^Si correction (triangles up) and after ew^Si and ew^Ca corrections (triangles down). The δA_U are displayed with an added arbitrary constant. The ellipses represent the full measured covariance matrix between the two bands. They are enlarged by the additional subtraction error after ew^Ca correction. The result of the fits are also displayed. The and respectively correspond to the ones in Figs. 3b and c for the U-band.