GRO J0422+32
GRO J0422+32 is an X-ray nova and black hole candidate that was discovered by the BATSE instrument on the CGRO satellite on 5 August 1992. During outburst, it was observed to be stronger than the Crab Nebula gamma-ray source out to photon energies of about 500 keV. The mass of the black hole in GRO J0422+32 falls in the range 3.66 to 4.97 solar masses. This is the smallest yet found for any stellar black hole, and near the theoretical upper mass limit (~2.7 ) for a neutron star. Further analysis  in 2012 calculated a mass of , which raises questions as to what the object actually is. It is also known to have a companion M-type main-sequence star, V518 Per, in the constellation Perseus. It has a magnitude of 13.5 in the B spectral band, and 13.2 in the visible band.