Radial velocities of Hβ and Hγ for the nights of 23 and 26 January 2004. Left panels: individual velocities of Hβ and Hγ for both nights plotted vs. orbital phase (top) and vs. spin phase (bottom). A typical error is shown attached to the open circle. The solid curves represent single sine fits. Center panels: mean radial velocities of Hβ and Hγ for 23 January (top) and 26 January (bottom), fitted by Eq. (7) (solid curve). The dashed and dash-dotted curves indicate the components that vary on the orbital and the spin periods, respectively. Right panels: same, but with the spin-modulated component subtracted.