NGC 4748. Upper panel: Broad Hβ component of a mock spectrum shifted redwards (red line) and bluewards (blue line) to mimic the orbital velocity of an equal-mass MBHB with orbital period τ_orb = 50 yr. The green line shows the composite line associated to the mock MBHB. Lower panel: Composite broad Hβ lines for mock MBHBs with τ_orb = 50, 100, 300, and 1000 yr (green, red, blue, and magenta lines, respectively). The Doppler shifts are evaluated assuming an edge-on circular binary with the MBH velocities perfectly aligned with the line of sight.