GR Muscae, also known as 2S 1254-690 is a binary star system in the constellationMusca composed of a neutron star of between 1.2 and 1.8 times the mass of the Sun and a low-mass star likely to be around the mass of the Sun in close orbit.[2] A magnitude 19 blue star was pinpointed as the optical counterpart of the X-ray source in 1978.[3] Its apparent magnitude varies from 18 to 19.1 over a period of 0.16 days.[4]
While the optical counterpart to the X-ray source was identified in 1978, optical variability was not detected until 1980, when a bright flare was seen that increased the white-light flux by a factor of two in a time interval of about 1.7 seconds.[5] GR Muscae received its variable star designation in 1985.[6]
The neutron star has an accretion disk that takes around 6.74 days to complete a revolution, and is inclined at an angle to the incoming stream of material from the donor star.[2]