Left: illustration of the assumed geometry of BD+46°442  for the Doppler tomography in the co-rotating frame of the binary. The mass centre of the binary coincides with the origin, and the blue dashed curve are the Roche lobes of the binary. The primary is located on the positive x-axis, and the position of the secondary is indicated with the cross on the negative x-axis. The purple curve represents the gas stream from the L1-point towards the secondary. The accretion disk is centred on the secondary. Right: system represented in velocity coordinates. The primary is represented as the full black circle, located on the positive V_y-axis. The location of the secondary is indicated by the cross on the negative V_y-axis. The curve originating from the primary indicates the actual velocity of the mass stream. The curve that originates at a higher V_y-velocity indicates the Keplerian velocities at the path of the mass stream. The velocities of the accretion disk are represented by the dashed and dotted circles, centred on the secondary. The four circles represent, from outside to inside, a projected Keplerian velocity of 150, 100, 75, and 50 km s^-1, respectively.