Mass dependence of the boost factor. Shown is the boost factor of the collinear configuration of a binary with a total mass of 2 M_⊙ with different mass ratios as a function of the internal separation: 2.0-0.0 M_⊙ (black), 1.5-0.5 M_⊙ (green), 1.9-0.1 M_⊙ (blue), and 1.99-0.01 M_⊙ (red). The equal-mass binary is plotted with grey squares. The configurations with the less massive component between the higher-mass component and the Galactic centre particle are indicated by triangles, while those where the less-mass component is on the side of the Galactic anti-centre are indicated by circles.