Gaia DR2 reduced proper motion versus colour diagram and reduced proper motion frequency plot used to define the selection criteria for the objects with a parallax error ≥ 20%. Left panel: reduced proper motion (H) versus colour (G_BP − G_RP) diagram for the blue horizontal-branch candidates with parallax error < 20%. Right panel: distribution of reduced proper motion (H) seen in the blue horizontal-branch candidates with a parallax error < 20%.