Posterior distributions of the additional noise terms (upper panels) and instrument offsets (lower panels) for HD 191806 under two competing models: with (right column) and without (left column) a secular acceleration term (order m in Eq. (2)). In the upper panels, we depict the posterior distribution of additional noise at activity minimum for SOPHIE and SOPHIE+ (see Sect. 5.1) and the amplitude of the (constant) additional noise for ELODIE. In the lower panels, the dotted curves correspond to prior distributions.