CHEOPS, HARPS-N and TESS: a joint effort to characterize the unexpected planetary system around TOI-561

Well-characterized multiplanetary systems offer a unique opportunity for comparative planetology, allowing for investigations of the formation and evolution processes. I will present the analysis of a new multiplanetary system, TOI-561, initially discovered by the TESS mission through photometric observations, and further characterized using high-precision RV data collected with the HARPS-N spectrograph, that allowed us to obtain precise masses for all the four planets in the system, namely an ultra-short period super-Earth and three mini-Neptunes (Lacedelli et al. 2021).  I will show how in this case combining photometric and RV data has been essential to discriminate between various possible architectures, highlighting the importance of combining different techniques to obtain a better characterization of a planetary system. Additional insights on the planetary architecture and internal structure are coming from the ultra-high precision photometry of the CHEOPS satellite (Lacedelli et al., in prep), which additionally stresses the importance of synergies between various techniques and instruments.