Relativistic mergers of black hole binaries have large, similar masses,
  low spins and are circular
  Gravitational waves are a prediction of general relativity, and with
ground-based detectors now running in their advanced configuration, we will
soon be able to measure them directly for the first time. Binaries of
stellar-mass black holes are among the most interesting sources for these
detectors. Unfortunately, the many different parameters associated with the
problem make it difficult to promptly produce a large set of waveforms for the
search in the data stream. To reduce the number of templates to develop, one
must restrict some of the physical parameters to a certain range of values
predicted by either (electromagnetic) observations or theoretical modeling. In
this work we show that "hyperstellar" black holes (HSBs) with masses $30
\lesssim M_{\rm BH}/M_{\odot} \lesssim 100$, i.e black holes significantly
larger than the nominal $10\,M_{\odot}$, will have an associated low value for
the spin, i.e. $a<0.5$. We prove that this is true regardless of the formation
channel, and that when two HSBs build a binary, each of the spin magnitudes is
also low, and the binary members have similar masses. We also address the
distribution of the eccentricities of HSB binaries in dense stellar systems
using a large suite of three-body scattering experiments that include
binary-single interactions and long-lived hierarchical systems with a highly
accurate integrator, including relativistic corrections up to ${\cal
O}(1/c^5)$. We find that most sources in the detector band will have nearly
zero eccentricities. This correlation between large, similar masses, low spin
and low eccentricity will help to accelerate the searches for
gravitational-wave signals.
