Comparison between fitted single star models and the ‘true’ values of the mass-gainer model for X_c (upper panel) and mass (lower panel) as a function of X_c, from the end of accretion until the end of the main sequence. Assuming single star evolution for stars that have accreted mass can lead to an overestimation of the central hydrogen fraction, X_c, and an underestimation of mass when not accounting for full multimodal posteriors. Each column shows, for a given X_c, a contour plot with the minimum χ² values (similar to Fig. 6) annotated with markers for the true value and absolute best fit.