Velocities obtained from H_β fitting (thin lines) compared to averages v₄ defined by Eq. (6). Heavy errorbars are rms deviations from v₄. Thin errorbars (shifted by 0.1 to the right for clarity) mark the standard error in a single H_β measurement estimated in Sect. 3.1 (15 km s^-1). Objects suspected of being velocity-variable (see Sect. 3.2) are indicated with an (s). All velocities are given in the heliocentric frame.