  Context. Periodic QSOs are considered as candidates of supermassive binary
black hole (BBH) systems in galactic centers. Further confirmation of these
candidates may require different lines of observational evidences. Aims.
Assuming the Doopler boosting scenario, in this paper we investigate the
(coherent) variations of both broad emission lines (BELs) and continuum light
curves for active BBH systems surrounding by a circumbinary broad line region
(cBLR) and focus on their dependence on the eccentric orbital configuration.
Methods. We calculate the variation of continuum light and the Doppler
enhanced/weakened photoionization of each BLR cloud according to the motion of
BBHs in elliptical orbits, and finally obtain the coherent variation of the
continuum and BELs. Results. We find that both the amplitude and variation
pattern of the continuum light curves and the evolution of the BEL profiles
sensitively depend on the eccentric orbital configuration of BBH systems. If
only the secondary BH is active, both the variation amplitudes of continuum
light curves and BELs increase with increasing BBH inclination angles and
orbital eccentricities, but decrease with increasing BBH mass ratio. If both
BHs are active, the asymmetry in the ionization of BLR clouds at different
areas caused by the Doppler boosting effect of the secondary BH is weakened due
to that of the primary BH at the opposite direction, which leads to
systematically smaller variation amplitudes of both continuum light curves and
BELs than the cases with only secondary BH activated. Conclusions. The coherent
variations of the BEL profiles with the continuum light for those periodic QSOs
provide an important way to confirm the existence of BBHs in their center.
Future joint analysis of the light curves and multi-epoch observed BEL profiles
for periodic QSOs may lead to the identification of a number of BBH systems.
