  Here we consider the perspective to detect sub-pc super-massive binary
black-hole (SMBBH) systems using long-term photometric and spectroscopic
monitoring campaigns of active galactic nuclei. This work explores the nature
of long-term spectral variability caused by the dynamical effects of SMBBH
systems. We describe in great detail a model of SMBBH system which considers
that both black holes have their accretion disc and additional line emitting
region(s). We simulate the H$\beta$ spectral band (continuum+broad H$\beta$
line) for different mass ratios of components and different total masses of the
SMBBH systems ($10^6-10^8\mathrm{M\odot}$). We analyze the set of continuum and
broad line light curves for several full orbits of SMBBHs with different
parameters, to test the possibility to extract the periodicity of the system.
We consider different levels of the signal-to-noise ratio, which is added to
the simulated spectra. Our analysis showed that the continuum and broad line
profiles emitted from an SMBBH system are strongly dependent, not only on the
mass ratio of the components but also on the total mass of the system. We found
that the mean broad line profile and its rms could indicate the presence of an
SMBBH. However, some effects caused by the dynamics of a binary system could be
hidden due to a low signal-to-noise ratio. Finally, we can conclude that the
long-term AGN monitoring campaigns could be beneficial for the detection of
SMBBH candidates.
