Effect of inclination. A: Fraction of axes at inclination ϕ < i vs i, assuming random inclinations. The red and blue dots mark two inclinations used in the discussion. B: Same as for A) but vs v sin i, adopting equatorial v = 100 km/s. C: Fraction of v sin i values for equidistant bins d v sin i = 1 km/s. The red and blue numbers give the cumulative contributions of the red and blue shaded area. D: Adopted constant distribution of stars with equatorial v_eq between 100 and 200 km/s (red shaded rectangle) and v sin i distribution resulting for random inclinations (blue histogram). The two black curves are the distribution in panel C scaled in v for 100 and 200 km/s. The blue histogram is constructed from the integration of all such black curves, it is scaled to the same area as the red histogram. About 30% of the stars from the rectangular red histogram will be observed with v sin i< 100 km/s. E: Same as for D) but separated with an inclination cut i > 30° (blue) and i < 30° (black). F: Same as for E) but adopting a triangular distribution of equatorial v between 50 and 350 km/s (red shaded). G: Same as for F) but allowing for a deviation between i_orb and i_rot with a tolerance of 30° for co-axial rotation (Sect. C.3). The last two panels show the rotational velocity distribution of stars in close SB2 systems with known orbital inclination, without and with inclination correction (middle panel: v sin i, right: v equatorial).