Color–magnitude diagram of the coronal eRASS1 sources. The color scales with X-ray luminosity. Magenta and black sources are flagged as noncoronal emitters; the magenta sources are known accreting or extragalactic objects in the Simbad database. Sources in the area outlined by the dashed blue line are flagged as OB_STAR. The blue and red lines show the PARSEC isochrones for stellar ages of 4 × 10⁷ and 4 × 10⁹ yr (Bressan et al. 2012), respectively, while the solid and dashed green lines in the inset show the 1.3 × 10⁸ yr isochrone for single and binary stars.