Comparison with other catalogues (2). HCO+/CO(1–0) line ratio versus HCN/CO line ratio. The two detections presented here (M31I and M31I-B) are superimposed on previous measurements, namely: detections across M31’s main disc by Brouillet et al. (2005) and a compilation of various types of galaxies from Krips et al. (2010). The ¹²CO(2–1) measurements presented here have been corrected by a CO 2–1/1–0 line ratio of 0.8, as measured in Melchior & Combes (2011) in a complex M31G within 10 and 30″ from the observed positions. The line indicates the best fit provided by Brouillet et al. (2005) for M31’s main disc. Our points are systematically above the best fit they provide, suggesting an excess of HCO+. The overall distribution of Krips et al. (2010) does not exhibit any clear pattern.