HD 108236 system observed with CHEOPS and TESS

We carried out a timing analysis of the transits of each of the planets of HD 108236 to search for the presence of transit timing variations. We derived improved values for the radius and mass of the host star (R_star_=0.876+/-0.007R_{sun}_ and M_star_=0.867_-0.046_^+0.047^M_{sun}_). We confirm the presence of the fifth transiting planet f in a 29.54d orbit.

Thus, the HD 108236 system consists of five planets of R_b_=1.587+/-0.028, R_c_=2.122+/-0.025, R_d_=2.629+/-0.031, R_e_=3.008+/-0.032, and R_f_=1.89+/-0.04[R_Earth_]. We refine the transit ephemeris for each planet and find no significant transit timing variations for planets c, d, and e. For planets b and f, instead, we measure significant deviations on their transit times (up to 22 and 28min, respectively) with a non-negligible dispersion of 9.6 and 12.6min in their time residuals.

We confirm the presence of planet f and find no significant evidence for a potential transiting planet in a 10.9d orbital period, as previously suggested. Further monitoring of the transits, particularly for planets b and f, would confirm the presence of the observed transit time variations. HD 108236 thus becomes a key multi-planetary system for the study of formation and evolution processes.

The reported precise results on the planetary radii -- together with a profuse RV monitoring -- will allow for an accurate characterization of the internal structure of these planets.