One of the components of the A0620-00 binary system is known to be a dark, compact stellar object, with a mass between nine and thirteen times the mass of the Sun. The other component is observed to be a K-type star (approximately 0.5 solar masses).
^Gelino, Dawn M., et al. "A Multiwavelength, Multiepoch Study of the Soft X-Ray Transient Prototype, V616 Monocerotis (A0620-00)". The Astronomical Journal, Volume 122, Issue 5, pp. 2668-2678, November 2001.