Hβ (left panel) and Hα (right panel) lines of star WD 1840−111 observed with ISIS in the blue and red arm, respectively. Top panel: I profile (in arbitrary units). Second panel from top: V/I profile. Third panel from top: x = −C_z λ²(1/I)dI/dλ vs. wavelength. The green vertical lines show the wavelength intervals in which the x values clearly depart from zero, and which have been choosen to apply the least-square technique. Bottom panel: relation between V/I and x for the points within the solid green lines in the upper panels. Note that the x and V/I ranges of the Hβ plots are half the size of the corresponing ranges in the Hα plots.