Median Ca II K line profiles of the gas-bearing debris-disc stars with non-photospheric absorption (black solid line) together with a Kurucz photospheric model (black dotted line) with stellar parametersaccording to Table 2. Dashed lines in all panels indicate the corresponding radial velocities of the stars (red), and the projected velocities of the local ISM (grey) after the Colorado model (Redfield & Linsky 2008). Velocities are in the reference frame of the Ca II K line. v_rad and v_ISM are coincident for HD 9672 (see Table 3). HD 181296 has no ISM lines in the line of sight (see Table 3 and Sect. 5.2 for a discussion). The same convention for v_rad and v_ISM is used in all further figures.