Fig. A.1
Measurements of the residual q,u
of standard stars after subtraction of their literature values (Table 1 circles: each color represents a different star). Blue star: weighted mean of 
excluding BD+33 2642. Purple star: weighted mean of BD+33 2642 measurements corrected using the literature value in the V -band (Table A.1). The black “×” marks the weighted mean found for DP2 in Sect. 3.
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