Now, an international team of astronomers has trained the Fermi space-based gamma ray telescope on V407 Cygni, a "binary" system comprising a white dwarf star and a red giant companion, 9, 000 light-years away.

BBC: Gamma rays from nova explosion surprise astronomers

The black holes are separated by an estimated distance of less than a third of a light-year - cheek-to-cheek by black hole standards and significantly closer than the postulated binary system spotted by the Chandra X-ray Observatory in 2003.

BBC: Dancing black hole twins spotted