Retrieved atmospheric C/O ratio, metallicity, and ¹²CO/¹³CO isotopologue ratio as functions of the planet mass (left) and stellar irradiation (right). The C/O ratios and metallicities of the companions (i.e. (C/O)_P and [Fe/H]_P) are given relative to that of their host stars ((C/O)_S and [Fe/H]_S), indicated as horizontal lines. Diamond markers indicate that the stellar C/O or [Fe/H] values were directly measured in previous studies (see Tables A.1 and A.2). We performed a linear fit of the relative atmospheric C/O ratio as a function of companion mass using orthogonal distance regression. The posterior median and confidence intervals are indicated by the solid line and shaded area. Although we used a linear function for the fit, the posterior median appears curved, which is indicative of the correlation between the slope and vertical offset of our fit. Arrows indicate lower bounds. The solar isotope ratio, i.e. ¹²C/¹³C= 89 (Anders & Grevesse 1989), is indicated by the dotted line, whilst the shaded region indicates the value measured for the local ISM, i.e. ¹²C/¹³C= 68 ± 15 (Milam et al. 2005). The bottom-left panel shows a scatter plot of the retrieved C/O ratios and metallicities.