Since the first observed gravitational wave signals from black hole mergers these kind of binaries get observable for us the first time. To merge within the Hubble time via gravitational wave radiation, an isolated binary needs to be tight when the second black hole is formed. Mainly a common-envelope phase is assumed to lead to a significant in-spiral and being therefore responsible for the formation of two compact objects in such a close binary. Similarly tight binaries of neutron stars or white dwarfs where the progenitors size would have been larger than their todays observed orbit experienced probably a common-envelope phase. However, for population synthesis calculations a common-envelope phase is a main reason of uncertainties. For example, the gravitational wave merger delay time after the formation of the second compact object strongly depends on the orbital separation which is highly influenced by the last common envelope and possible kick during its formation. 

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