Mass–radius diagram for LP 791-18 d (square) and c (triangle), with updated measurements from this work (filled markers from the damped eccentricity TTV results, open markers for the free eccentricity TTV results) compared to the mass constraints from P23 (gray markers). Filled circles represent all small (<3 R_⊕) planets orbiting M dwarfs (T_* < 3900 K) with masses and radii measured to better than 3σ, based on the NASA Exoplanet Archive list of confirmed planets as of 2024 November. The predicted equilibrium temperatures of the planets are indicated by the point color. For comparison, we also plot Earth-like water-rich mass–radius curves (for 10, 30, and 50% water mass fractions) from A. Aguichine et al. (2021) and pure iron, Earth-like, and rocky iso-density curves from L. Zeng et al. (2016).