Top: variations of the relative difference of the abundances produced by the line-ratio method as a function of the rotational temperature Δd (T_rot) in the case of AI CMi and R Sct. The minimum relative difference corresponds to the best-fit value of T_rot. This procedure can only be performed for those sources in which several transitions of the same molecule have been observed. See text and Appendix B for more details. Bottom: variations of the relative error of the observational integrated intensity line and the predicted one as a function of the rotational temperature for SiO J = 5–4. The predicted integrated intensity lines are derived from Eq. (B.11) using the estimated abundance and rotational temperature.