  Observations have definitively strengthened the long-standing assertion that
binaries are crucial in massive star evolution. While the percentage of
spectroscopic binary systems among main-sequence O stars is well-studied, other
phases of massive star evolution remain less explored. We aim to estimate the
spectroscopic binary fraction in Galactic late O- and B-type supergiants
(OB-Sgs) and set empirical thresholds in radial velocity (RV) to avoid
misidentifying pulsating stars as single-line spectroscopic binaries. Using
over 4500 high-resolution spectra of 56 Galactic OB-Sgs (plus 13 O
dwarfs/subgiants and 5 early-B giants) from the IACOB project (2008-2020), we
apply Gaussian fitting and centroid computation techniques to measure RV for
each spectrum. Our findings reveal that intrinsic variability in OB-Sgs can
result in peak-to-peak RV amplitudes (RVpp) of up to 20-25 km/s, notably in
late-O and early-B Sgs, and decreases to typical values of RVpp in the range of
1-5 km/s for O dwarfs and 2-15 km/s for late B-Sgs. Considering these results
and evaluating line-profile variability in each star, we find that 10$\pm$4% of
OB-Sgs are clearly single-line spectroscopic binaries. In addition, we find
that the percentage of double-line spectroscopic binaries in late O- and early
B-Sgs is ~6%, much lower than the ~30% in O-type dwarfs and giants. This study,
along with prior research on B-Sgs in the 30 Doradus region of the LMC,
indicates that the spectroscopic binary percentage decreases by a factor of 4-5
from O stars to B-Sgs. Our study underscores the need for a thorough
characterization of spectroscopic variability due to intrinsic sources to
reliably determine the spectroscopic binary fraction among OB-Sgs and O-type
stars in general, offering valuable insights into the impact of binaries on
massive star evolution.
