Ratio of the mass and rotational temperatures for the hot and warm components, , for sources with a double-T_rot component, namely, the AGB stars NML Tau, π Gru, W Aql, and χ Cyg and the yPN IRAS 17347-3139. The solid line is a fit by a power-law function (y ∼ x^−1/α) to the AGB stars’ data – see text in Sect. 7.1. A similar trend is found in our analogue study of the THROES C-rich sample reported in Paper II.