Predicted distribution of OB stars with a NS (left panel) and BH (right panel) companion in the Hertzsprung–Russell diagram. The green line is the ZAMS, and the black line is the terminal-age main sequence (TAMS). The blue lines represent evolutionary tracks of non-rotating core hydrogen burning single stars, with the indicated initial masses. The dashed red lines show the boundaries of the three groups of stars defined in the main text, i.e. T_eff = 31.6 kK, log L/L_⊙  = 5, and the MS evolutionary track of a 8.9 M_⊙ single star. On top, we label several spectral types at their approximate effective temperature.