  The recently detected gravitational wave signals (GW150914 and GW151226) of
the merger event of a pair of relatively massive stellar-mass black holes (BHs)
calls for an investigation of the formation of such progenitor systems in
general. We analyse the common envelope (CE) stage of the "traditional"
formation channel in binaries where the first-formed compact object undergoes
an in-spiral inside the envelope of its evolved companion star and ejects the
envelope in that process. We calculate envelope binding energies of donor stars
with initial masses between 4 and 115 Msun for metallicities of Z=Zsun/2 and
Z=Zsun/50, and derive minimum masses of in-spiralling objects needed to eject
these envelopes. We find that CE evolution, besides from producing WD-WD and
NS-NS binaries, may, in principle, also produce massive BH-BH systems with
individual BH component masses up to ~50-60 Msun, in particular for donor stars
evolved to giants. However, the physics of envelope ejection of massive stars
remains uncertain. We discuss the applicability of the energy-budget formalism,
the location of the bifurcation point, the recombination energy and the
accretion energy during in-spiral as possible energy sources, and also comment
on the effect of inflated helium cores. Massive stars in a wide range of
metallicities and with initial masses up to at least 115 Msun may possibly shed
their envelopes and survive CE evolution, depending on their initial orbital
parameters, similarly to the situation for intermediate mass and low-mass stars
with degenerate cores. We conclude that based on stellar structure
calculations, and in the view of the usual simple energy budget analysis,
events like GW150914 and GW151226 could possibly be produced from the CE
channel. Calculations of post-CE orbital separations, however, and thus the
estimated LIGO detection rates, remain highly uncertain. [Abridged]
