Nova Muscae 1991 is an X-ray binary believed to be composed of a low-mass late-type companion that is orbiting a massive object, possibly a black hole. The nova event detected in 1991 both as a transient X-ray source and as an increase in optical brightness by about eight magnitudes. The tremendous brightening of the source probably arose from the sudden accretion of material from the low mass star onto its invisible companion. Recent optical photometric and spectroscopic observations reveal an orbital period of 10.4 hours. The low mass K0-K4 star is orbiting around an invisible companion with a maximum observed velocity of 409 km/s. Because the inclination of the orbit to the plane of the sky is unknown, the mass of the compact companion cannot be determined for sure. However, it is quite likely that the invisible star is at least 3 solar masses and therefore might be a black hole.

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