Auto-correlation functions of 144 RV data points of a rotating star (P_rot = 25 days) with one single central spot. We show the dependence of K_ACF on stellar inclination angles i = 30, 45, 60, and 90 deg with the colours as indicated. The dependence on the radius of the dark spot R = 5, 10, 20, and 40 deg is illustrated by the different symbols as indicated. We show the repetitive part of the ACFs with τ < P_rot.