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Stellar evolution and black hole formation

The evolution of the black hole transient sources is sketched. These contain a black hole of mass ∼7M⊙ with a low-mass companion star. In five of the six binaries in which the companion is in main sequence, it is a K-star; in the other case, an M-star. In three other cases the companion is a more massive star in subgiant phase. The near equality in main sequence masses of the companions shows that they originate at a well defined initial separation ai, also that there cannot be a wide variation in main sequence mass of the black hole progenitor. We make the case that these transient sources are relics of gamma ray bursters.