Difference between observed (O) and calculated (C) mid-transit times for TOI-421 b (top) and TOI-421 c (bottom) using observations by TESS (blue) and CHEOPS (green). The red line indicates a difference of zero, which corresponds to no TTVs. The purple lines indicate the 1σ uncertainties of the linear ephemeris. The red area represents the amplitude of the expected variations considering the dynamical interactions of the two planets as computed from the TTVfast code. See text for further details.