Comparison with other catalogues (1). HCN/HCO+(1–0) line ratio versus ¹³CO/¹²CO(2–1) line ratio. The two north-west detections presented in this paper (M31I and M31I-B) together with the red south-east stacking measurement are superimposed on previous measurements, namely: a compilation of various types of galaxies from Krips et al. (2010), early-type galaxies from Crocker et al. (2012) and M33’s giant molecular clouds from Buchbender et al. (2013). The hatched area corresponds to the mean HCN/HCO+(1–0) line ratio obtained by Brouillet et al. (2005) in M31’s disc. Surprisingly, our detections, compatible with the M31 disc measurements of Brouillet et al. (2005), lie in “the peculiar/merger region”.