Primordial atmospheric (water vapour) mass fraction of TOI-238 b against present-day mass and radius of the planet, as given by the observational constraints. Four panels correspond to different values of the heating efficiency parameter η: 0.01 (a), 0.05 (b), 0.1 (c), and 0.15 (d). In all four cases, the planetary core has an Earth-like composition and the luminosity of 10²¹ erg/s/cm², and the star evolved as a slow rotator.