RVs of τ Ceti derived only using the spectral lines with wavelength longer than 5000 Å (left panel), shorter than 5000 Å (middle panel) and shorter than 5000 Å plus a quadratic drift fitted to the residual RVs to remove low frequencies (right panel). This quadratic drift corresponds to a small variation of the RVs of less than 1.5 m s⁻¹ over 11 years. It is clear that spectral lines with wavelengths longer than 5000 Å induce significant signals at periods between 100 and 1000 days. This is certainly due to telluric and micro-telluric lines affecting the red part of the spectrum.