Planetary mass versus C/O ratio for hot and/or ultra-hot Jupiters with constrained abundances of C- and O-bearing species from ground-based high-resolution observations. Our retrieved C/O values for MASCARA-1b are shown as diamonds (labelled M-1b). Values for other planets are from previous studies using either free-chemistry retrievals or chemical equilibrium models (see Table F.1). The dashed line indicates the solar C/O value, and Jupiter is included for reference (from Weiner Mansfield et al. 2024). To reduce clutter, the labels for planets excluding MASCARA-1b and WASP-18b are shown in the inset, which provides a zoomed-in view. For clarity, small mass offsets are applied to WASP-121b, WASP-76b, WASP-189b and MASCARA-1b.