Chromium-hydride (CrH) cross-correlation search and sensitivity check. The bottom panel shows the phase-folded K_p-V_sys signal-to-noise map of the ESPRESSO WASP-31b transit data cross-correlated with a CrH model template. The dashed lines denote the expected values of (V_sys = −0.1 km s⁻¹, and K_p = 148.0 km s⁻¹, Gaia Collaboration 2023; Flagg et al. 2023). The top panel shows a one-dimensional slice at the expected K_p (along the horizontal dashed white line) for both the data alone (blue) and the data with an injected model (orange). The difference between the blue and orange lines is a measure of the data sensitivity to an underlying signal. In this case, these data only have limited sensitivity to CrH, which has its strongest opacity band heads redward of the ESPRESSO wavelength coverage. Although no clear CrH signal is observed in the data here, we cannot conclusively confirm or rule out its presence in the atmosphere of WASP-31b due to limited sensitivity.