  Israelian et al. (1999) showed that the stellar companion of the black-hole
binary Nova Sco is polluted with material ejected in the supernova that
accompanied the formation of the black-hole primary. Here we systematically
investigate the implications of these observations for the black-hole formation
process. Using a variety of supernova models, including both standard as well
as hypernova models (for different helium-star masses, explosion energies, and
explosion geometries) and a simple model for the evolution of the binary and
the pollution of the secondary, we show that most of the observed abundance
anomalies can be explained for a large range of model parameters (apart from
the abundance of Ti). The best models are obtained for He star masses of 10 to
16 Msun, where spherical hypernova models are generally favoured over standard
supernova ones. Aspherical hypernova models also produce acceptable fits,
provided there is extensive lateral mixing. All models require substantial
fallback and that the fallback material either reached the orbit of the
secondary or was mixed efficiently with material that escaped. The black hole
therefore formed in a two-step process, where the initial mass of the collapsed
remnant was increased substantially by matter that fell back after the initial
collapse. This may help to explain the high observed space velocity of Nova Sco
either because of a neutrino-induced kick (if a neutron star was formed first)
or by asymmetric mass ejection in an asymmetric supernova explosion.
