V923 Aquilae

V923 Aquilae is a variable binary star system in the equatorial constellation of Aquila. It has the designation HD 183656 from the Henry Draper Catalogue; V932 Aql is the variable star designation. The system is dimly visible to the naked eye with an apparent visual magnitude that fluctuates around 6.06. It is located at a distance of approximately 890 light years from the Sun based on parallax, but is drifting closer with a radial velocity of around −26 km/s.

This system was first identified as a likely spectroscopic binary by W. E. Harper in 1937, who noted it showed "narrow intense lines of peculiar spectrum". P. W. Merrill and C. G. Burwell identified it as a shell star in 1949. Merrill and A. L. Lowen showed in 1953 that the shell displayed large radial velocity variations. A photometric study by C. R. Lynds in 1960 showed the system varied in brightness with an amplitude of more than 0.1 in magnitude and a characteristic period of 0.85 days, although it does not behave periodically over long time intervals.

A more thorough investigation by P. Koubský and associates in 1989 using long-term radial velocity measurements determined this is a spectroscopic binary with an orbital period of 214.75 days. There is also an overlaying long-term cyclical variation of changing amplitude and period. The modelled binary system shows a primary with a class of around B5–7e and a low mass secondary separated by around 250 times the radius of the Sun ($−0.019$). They hypothesized that the long-term variation was due to an envelope created by a mass transfer from the secondary component to the primary. However, the mass transfer concept was later brought into question and remains unverified as of 2004.