The spectral region around the three (4238.19, 4262.27 and 4297.06 Å) Tc I resonance lines in our sample of Tc-rich M stars. For comparison purposes, the spectrum of a Tc-poor S star (HD 233158, in green in the top panels) from S18 and a Tc-rich S star (V812 Oph, in red) from S21 are also plotted. The spectra have been arbitrarily normalized and smoothed using a Gaussian filter with a standard deviation (sigma) of 1Å to slightly increase the S/N ratio.