In the constellation Monoceros, an X-ray source known as A0620-00 brightened by more than five orders of magnitude (100,000 times) in the winter of 1975. This X-ray nova event is associated with a main sequence K star known to show optical brightness variations and thus designated V616 Mon. The K star orbits an unseen companion once every 7.75 hours with a maximum velocity of 457 km/s. It is believed that the light of the K star varies because the star's shape is distorted by the gravitational pull of its invisible but massive companion. The mass of the compact star must be greater than 3.2 solar masses and may be greater than 7.3 solar masses, making this an excellent candidate for a black hole.

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