The black hole binary A0620-00

CCD photometry obtained with the 1.3-m McGraw-Hill telescope during 1981-1985 and 490-560-nm spectra obtained at KPNO on Jan. 17, 1985, are reported for A0620-00. The mass of the compact X-ray source is found to have lower limit 3.20 solar mass, strongly implying that it is a black hole; its mass is shown to exceed 7.3 solar mass if the K dwarf fills its Roche lobe during quiescence.