Radial velocity of the narrow component of the HeI 5876 Å line profile in the stellar rest frame plotted as a function of Julian date (top) and rotational phase computed from the ephemeris of Eq. (1), (bottom). The color code corresponds to successive rotational cycles. The fit by a geometrical accretion shock model (see text) is shown (dash-dotted curve) together with its 1σ uncertainty (gray area).