TOI-521 and TOI-912 RV datasets

The two stars host nearly identical planets in terms of mass and radius. TOI-521 hosts a transiting sub-Neptune in a 1.5-days orbit with radius and mass of R_b_=1.98+/-0.14R_{Earth}_ and M_b_=5.3+/-1.0M_{Earth}_, respectively. Moreover, we identified an additional candidate at 20.3-days, with a minimum mass of Mpsin(i)=10.7M_{Earth}_, currently not detected to transit in our photometric dataset.

Similarly, the planet orbiting TOI-912 is a 4.7-d sub-Neptune with R_b_=1.93+/-0.13R_{Earth}_ and M_b_=5.1+/-0.5M_{Earth}_. Interestingly, TOI-912 b likely possesses an unusually high eccentricity (e=0.58+/-0.02), and it is probably undergoing strong tidal dissipation. If such eccentricity is confirmed, it would make it one of the most eccentric sub-Neptunes known to date.

TOI-521 b and TOI-912 b have very similar densities (4g/cm^3^), and they lie in the degenerate region of the mass-radius diagram where different compositions are plausible, including a volatile-rich composition, or a rocky core surrounded by a H-He envelope. When compared to the other THIRSTEE M-dwarf targets, our sample supports the division of sub-Neptunes into two distinct populations divided by a density gap. Both planets are interesting targets for atmospheric follow-up in the context of understanding the temperature-atmospheric feature trend that starts to emerge thanks to JWST observations.