Central wavelengths of the Tc I blends at λ4238 and λ4262 Tc I. The dashed grey lines delineate the boundaries between the Tc-poor zone (upper left rectangle) and the Tc-rich region (lower-right rectangle, Van Eck & Jorissen 1999). Observed data points are shown as squares and triangles, corresponding to measurements of (λ₄₂₃₈, λ₄₂₆₂) in spectra of Tc-rich and Tc-poor M and S stars, as labelled. Synthetic spectra were generated for two effective temperatures (T_eff = 2800 K and 4000 K), two metallicities ([Fe/H] = 0.0 and –0.5), two microturbulence velocities (χ_t = 2 and 3 km/s), and a range of Tc abundances (as indicated by the colour scale on the side). From these synthetic spectra, the central wavelengths of the λ4238 and λ4262 Tc I blends were measured. The results are represented as coloured circles, connected by lines that group models with the same atmospheric parameters, as indicated by the labels.