Astrometric parameters of NGC 6611. Top-left: proper motion distribution of the field stars (grey) and members of NGC 6611 (blue). We show the O stars present in NGC 6611 with poor and good quality astrometry with the red squares and cyan circles, respectively. Top-right: parallax distribution of the field stars and members, coloured similarly as in the top-left panel. We also show the fractional parallax uncertainty (ϖ/σ_ϖ) of the members as a function of their parallax. Two O star systems have RUWE = 2.544 and 8.382, and have parallaxes outside this panel, with only part of the error-bar visible. The 1σ error on the parallax of NGC 6611 is shown with the orange bar. Bottom-left: radial velocity of the members in the heliocentric frame collected from the literature. Bottom-right: radial velocity in the heliocentric frame of the B0 to B4 stars after removal of the outliers in the bottom-left panel. Outliers have been excluded as described in Sect. 3. The mean and standard deviation of the radial velocity of these stars are shown with the dashed black line and shaded region, respectively.