H⁻/H₂O index strengths of XSL stars as a function of their (I − K) colours. Points are colour-coded by their effective temperatures from Arentsen et al. (2019), saturated at 10 000 K. Carbon stars are marked with circles. Supergiants are marked with diamonds. Both C-rich TP-AGB stars and supergiants are excluded from the division into static and variable stars. M dwarfs have weak H⁻/H₂O index strengths but can have red (I − K) colours, and they have been marked with triangles.