  We investigate on the spectral properties of an active black hole, member of
a massive (10^7 - 10^9 Msun) sub-parsec black hole binary. We work under the
hypothesis that the binary, surrounded by a circum-binary disc, has cleared a
gap, and that accretion occurs onto the secondary black hole fed by material
closer to the inner edge of the disc. Broad line emission clouds orbit around
the active black hole and suffer erosion due to tidal truncation at the Roche
Lobe surface, following gap opening and orbital decay. We consider three of the
most prominent broad emission lines observed in the spectra of AGNs, i.e. CIV,
MgII and H{\beta}, and compute the flux ratios between the lines of MgII and
CIV (FMgII/FCIV) and those of MgII and H{\beta} (FMgII/FH{\beta}). We find that
close black hole binaries have FMgII/FCIV up to one order of magnitude smaller
than single black holes. By contrast FMgII/FH{\beta} may be significantly
reduced only at the shortest separations. Peculiarly low values of line flux
ratios together with large velocity offsets between the broad and narrow
emission lines and/or periodic variability in the continuum (on timescales >=
years) would identify genuine sub-pc binary candidates.
