BEL profile variations in a single orbital period for BBHs co-rotating in circular orbits with i_obs = 60°. The two left columns show BBH systems when only the secondary BH is active, i.e., q_L = 1 : 0, but with q_M = 0.1 (left column) and 0.5 (middle-left column). The two right columns show profile variations when both the two BBHs are active with q_L = 1 : 1, in the case of q_M = 0.1 (middle-right column) and 0.5 (right column). In each column the top panel shows BEL profiles of the six phases (0 to 5π/3) in one period color-coded from blue to red (see legend), and the bottom panel presents the relative flux variation of BEL profiles compared to the mean profile obtained from that of the six phases.