  The first directly detected gravitational waves (GW 150914) were emitted by
two coalescing black holes (BHs) with masses of ~36Msun and ~29Msun. Several
scenarios have been proposed to put this detection into an astrophysical
context. The evolution of an isolated massive binary system is among commonly
considered models. Various groups have performed detailed binary-evolution
calculations that lead to BH merger events. However, the question remains open
as to whether binary systems with the predicted properties really exist. The
aim of this paper is to help observers to close this gap by providing spectral
characteristics of massive binary BH progenitors during a phase where at least
one of the companions is still non-degenerate. Stellar evolution models predict
fundamental stellar parameters. Using these as input for our stellar atmosphere
code (PoWR), we compute a set of models for selected evolutionary stages of
massive merging BH progenitors at different metallicities. The synthetic
spectra obtained from our atmosphere calculations reveal that progenitors of
massive BH merger events start their lives as O2-3V stars that evolve to
early-type blue supergiants before they undergo core-collapse during the
Wolf-Rayet phase. When the primary has collapsed, the remaining system will
appear as a wind-fed high-mass X-ray binary. We provide feedback parameters,
broad band magnitudes, and spectral templates that should help to identify such
binaries in the future. Comparisons of empirically determined mass-loss rates
with those assumed by evolution calculations reveal significant differences.
The consideration of the empirical mass-loss rates in evolution calculations
will possibly entail a shift of the maximum in the predicted binary-BH merger
rate to higher metallicities, that is, more candidates should be expected in
our cosmic neighborhood than previously assumed.
