Distribution of proper motion in Galactic longitude as a function of colour for the ϖ-background sources in Baade’s window. The red curve is the median value of μ_ℓ* for each bin in (g − i)₀, and the width of the red band is computed using the 16th and the 84th percentiles of the distribution of proper motions in each bin. The black dashed horizontal line corresponds to the mean of the Gaussian fitted to the distribution of the bulge stars.