Mass discrepancy plotted versus luminosity for only the single stars. Closed symbols are those with effective temperature <20 000 K and open symbols are singles with T_eff≥ 20 000 K. The upper dashed blue line is from the L(M) relation of helium stars: a star of a given mass cannot be more luminous than this. The lower blue line is M_e = M_s, and is a lower limit as stars cannot be under luminous.