Colour−magnitude (left) and colour–colour (right) diagram. Black points show all possible optical counterparts of X-ray sources with measured U, V, and B magnitudes in the MCPS found inside the 3σ positional uncertainty. Counterparts for Galactic foreground star candidates were selected redwards of the blue dashed line only. The red boxes mark the selection region of counterparts for BeXRBs in both plots. Identified foreground stars (filled orange stars), classified foreground stars (open orange stars), identified BeXRBs (blue squares) are marked. WR stars in the SMC and SMC X-1 are shown by cyan stars and the magenta square. Red dots in the right diagram mark all sources (black dots) within the red box of the left diagram. The green line gives the colours for the unreddened main sequence according to Fitzgerald (1970).