First panel: astrometric u parameter as a function of G magnitude. The red dashed line is the u selection criterion from Arenou et al. (2018). Second panel: shows how this factor depends on RA. The u parameter is affected by crowding in the central parts of the ONC. Third panel: photometric excess, E, parameter as a function of the color in Gaia DR2 filters. The region between the dashed red lines shows the E selection criterion from Arenou et al. (2018). Fourth panel: E as a function of RA. demonstrating that E and therefore Gaia DR2 photometry in bp and rp filters are significantly affected by crowding. In all panels, the black points are the data from the initial catalog.