DELLA VALLE, MassimoMassimoDELLA VALLEJarvis, B. J.B. J.JarvisWest, R. M.R. M.West2022-09-282022-09-2819910028-0836http://hdl.handle.net/20.500.12386/32661X-RAY novae form a subclass of low-mass X-ray binaries, which typically consist of a neutron star accreting material from a low-mass, late-type companion. In a few cases, however-notably A0620-00 (ref. 1)-, the accreting object may be a black hole. The detection and study of the optical counterpart is then of great importance in understanding the origin of the X-ray emission and the nature of the companion. We report here the discovery of a new X-ray nova, Nova Muscae 1991, which we found in a search for the optical counterpart of the new transient X-ray source GRS1121-68. The optical position, light-curve and early spectral development of the outburst support the identification of Nova Muscae 1991 as a binary, and similarities with A0620-00 (known optically as V616 Mon) in its early evolutionary stages led us to classify Nova Muscae 1991 as a candidate black-hole binary.STAMPAenArticle10.1038/353050a02-s2.0-0002095537https://api.elsevier.com/content/abstract/scopus_id/0002095537https://www.nature.com/articles/353050a01991Natur.353...50DFIS/05 - ASTRONOMIA E ASTROFISICA