Here aB is calculated from Kepler’s third law for each pair of MB (B-star mass) and MBH (black-hole mass). The contours and colours both represent the values of aB. The white dashed lines in the contour plot outline a valid region for the semi-major axis of the B star. It comes from the limitations on MB, (7–9.1)M☉, and on MBH, (55–79)M☉. See Methods for details.