Illustration of the Tc lines in a selection of stars. Left panel: 4238.191 Å Tc I line. Middle panel: 4262.270 Å Tc I line. Right panel: 4297.058 Å Tc I line. A dashed vertical line at each panel centre marks the laboratory wavelength of the Tc transitions. The stars from top to bottom are R Leo, R LMi, U Ari, S CMi, o Cet, R Lyn, and WY Cas. Spectra of stars classified as Tc-poor are plotted as blue graphs, and those classified as Tc-rich as red graphs. For clarity, the spectra are shifted by multiples of 0.4 arbitrary flux units along the y-axis.