Time series of the Stokes I profiles for the 2010 observations of EV Lac computed with (Left) the full mask without telluric window a priori, (Middle) union of the 2010-trained best sub-masks, and (Right) the 2010-trained worst RMS-based sub-mask without telluric windows a priori. The Stokes profiles are shifted vertically for visibility, and sorted by ascending observation date. An increased sharpness can be observed when the union of best sub-masks is used, and a broad, distorted profile when the worst sub-mask is used, implying the presence of stable and unstable lines in the sub-masks, respectively.