The TOI-421 planetary system contains two sub-Neptune-type planets (Pb~5.2-days, T_eq,b_~900K, and Pc~16.1-days, T_eq,c_~650K) and is a prime target to study the formation and evolution of planets and their atmospheres. The inner planet is especially interesting as the existence of a hydrogen-dominated atmosphere at its orbital separation cannot be explained by current formation models without previous orbital migration. We aim to improve the system parameters to further use them to model the interior structure and simulate the atmospheric evolution of both planets, to finally gain insights into their formation and evolution. We also investigate the possibility of detecting transit timing variations (TTVs). We jointly analysed photometric data of three TESS sectors and six CHEOPS visits as well as 156 radial velocity data points to retrieve improved planetary parameters. We also searched for TTVs and modelled the interior structure of the planets. Finally, we simulated the evolution of the primordial H-He atmospheres of the planets using two different modelling frameworks. We determine the planetary radii and masses of TOI-421 b and c to be R_b_=2.64+/-0.08R_{Earth}_, M_b_=6.7+/-0.6M_{Earth}_, R_c_=5.09+/-0.07R_{Earth}_, and M_c_=14.1+/-1.4M_{Earth}_. Using these results we retrieved average planetary densities of rho_b_=0.37+/-0.05rho_{Earth}_ and rho_c_=0.107+/-0.012rho_{Earth}_. We do not detect any statistically significant TTV signals. Assuming the presence of a hydrogen-dominated atmosphere, the interior structure modelling results in both planets having extensive envelopes. While the modelling of the atmospheric evolution predicts for TOI-421 b to have lost any primordial atmosphere that it could have accreted at its current orbital position, TOI-421 c could have started out with an initial atmospheric mass fraction somewhere between 10 and 35%. We conclude that the low observed mean density of TOI-421b can only be explained by either a bias in the measured planetary parameters (e.g. driven by high-altitude clouds) and/or in the context of orbital migration. We also find that the results of atmospheric evolution models are strongly dependent on the employed planetary structure model.

A. F. Krenn et al. -- We present CHEOPS photometry extracted using PIPE (raw and detrended) of TOI-421 containing four transits of planet b and 2 transits of planet c. Further we present detrended TESS photometry of TOI-421. We also present raw and detrended HARPS radial velocities of TOI-421 reduced by fitting a skew normal fit to the CCFs. Finally, we also include raw and detrended radial velocities of TOI-421 observed by FIES, PFS and HIRES. The raw data was originally published by Carleo et al. (2020AJ....160..114C, Cat. J/AJ/160/114). ch1.dat contains the raw CHEOPS photometric time series with auxiliary detrending vectors. ch2.dat contains the detrended CHEOPS light curve. te1.dat contains the detrended TESS light curve. harps.dat, hires.dat, fies.dat and pfs.dat contain the HARPS, HIRES, FIES and PFS raw and detrended (including offset-corrected) radial velocities.

Peer reviewed