Doppler maps of the SDSS J0039 system for the Hα (left), He I 6678 Å (centre) and He I 7065 Å (right) emission lines. The Roche lobe of the mass donor is indicated by a solid curve and the centre of mass of the system by a cross. The upper dots indicate the velocity of the accretion stream and the lower dots the Keplerian velocity of the disc along the path of the stream. They are positioned at every 0.1   R_RL, decreasing from R_RL = 1.0 at the mass donor, where R_RL is the radius of the WD Roche lobe. This configuration corresponds to K_WD = 60 km s^-1 and K₂ = 400 km s^-1.