AC Herculis

AC Herculis, is an RV Tauri variable and spectroscopic binary star in the constellation of Hercules. It varies in brightness between apparent magnitudes 6.85 and 9.0.

AC Her is an RVa star, meaning it is an RV Tauri variable whose maximum and minimum magnitudes do not slowly vary over hundreds of days. It also is a very clear example of a common type of RV Tauri light curve where the maximum following a deep minimum is brighter than the maximum following a shallow minimum. In each period of 75.46 days it has two maxima and two minima.

AC Her is also a binary star, its companion was detected with spectroscopy and long baseline interferometry. Interferometric observations from the CHARA Array enabled its three dimensional orbit with a semimajor axis of 2.01 ± 0.01 mas, equivalent to 2.83 ± 0.08 au. The orbit has an inclination of 142.9 ± 1.1 degrees and a longitude of the ascending node of 155.1 ± 1.8 degrees. The invisible secondary is more massive than the supergiant primary. The orbital period is 1187.7 days. The two stars are also surrounded by a dusty disc filling the region between 34 and 200 astronomical units (AU).

Little is known of the secondary star except that its mass is around, deduced from the mass ratio of the binary system and the modelled mass of the primary star. The primary itself is calculated to have a mass of, but a luminosity of. It is slightly cooler than the sun, although this varies by over a thousand K as the star pulsates.

The total system mass can be estimated from the dynamics of the disc, and this gives a value of, slightly lower than from other methods.