ZrO band heads of five selected sample stars. The stars from top to bottom are R LMi, U Ari, ST Sgr, R Lyn, and WY Cas. Spectra of Tc-poor stars are plotted as blue and Tc-rich stars as red graphs. For clarity, the spectra are shifted by multiples of 0.5 arbitrary flux units along the y-axis. The wavelengths of ZrO band heads, taken from Davis (1947), are marked by dashed vertical lines. From left to right: ZrO γ(0, 0) Rc (6473.54 Å), γ(0, 0) Xc (6480.57 Å), and γ(1, 1) Rc head (6495.00 Å). ST Sgr is discussed in more detail in Sect. 3.2.