Abstract
Sub-Neptunes have been found to be one of the most common types of exoplanets, yet their physical parameters and properties are poorly determined and in need of further investigation. In order to improve the mass measurement and parameter determination of two sub-Neptunes, K2-266 d and K2-266 e, we present new transit observations obtained with CHaracterising ExOPlanets Satellite and Transiting Exoplanet Survey Satellite, increasing the baseline of transit data from a few epochs to 165 epochs for K2-266 d, and to 121 epochs for K2-266 e. Through a two-stage-fitting process, it is found that the masses of K2-266 d and K2-266 e are 6.01 +/- 0.43 M circle plus and 7.70 +/- 0.58 M circle plus, respectively. With these updated values and one order of magnitude better precision, we confirm the planets to belong to the population of planets that has been determined to be volatile-rich. Finally, we present the results of dynamical simulations, showing that the system is stable, the orbits are not chaotic, and that these two planets are close to but not in 4:3 mean motion resonance.