Abscissa residuals Δη (in mas on the sky; red dots) along with the corresponding error bar from van Leeuwen (2007a) for Betelgeuse and Vega, as a function of time, expressed in years from 1991.25. Filled star symbols correspond to the along-scan projections P_θ of the synthetic photocentre displacements of Fig. 3. The red dashed lines and black solid lines depict the  ± 1σ interval around the mean for the Hipparcos data points and model predictions, respectively. Note that these displacements were computed in the Gaia G filter instead of the Hipparcos Hp filter. A test on a given snapshot has shown that the difference is negligible: P_x = 0.11 AU with the Hp filter, as compared to 0.13 AU with the G filter.