Comparison of the orbital evolution of the SMBH binary computed in two ways: fully accounting for the disk evolution driven by the tidal torque of the binary (solid black curves) and simply assuming the disk properties to be given by a standard constant solution (dotted blue curves). In both cases, the values of , the initial semi-major axis of the binary, and all other parameters are assumed to be the same. Results are shown for M _c = 10 ⁵  M _☉, (panel (a)) and for M _c = 10 ⁷  M _☉, (panel (b)), with the binary mass ratio q = 1 in all cases. The meaning of other curves and labels on these plots is the same as in Figure  5. Note that properly accounting for the binary-driven disk evolution can reduce the lifetime of the binary by almost an order of magnitude in some cases.