Phi Herculis

Phi Herculis is a binary star system in the northern constellation of Hercules. Based upon an annual parallax shift of 15.99 mas as seen from Earth, it is located around 204 light years from the Sun. With a combined apparent visual magnitude of 4.24, it is bright enough to be seen with the naked eye.
File:PhiHerLightCurve.png|thumb|left|250px|A light curve for Phi Herculis, assuming a 3.708 day period, plotted from Transiting Exoplanet [Survey Satellite|TESS] data
This is a single-lined spectroscopic binary star system with an orbital period of 564.8 days and an eccentricity of 0.526. The primary, component A, is a B-type [main sequence star] with a stellar classification of B9VspHgMn. It is a chemically peculiar star of the type called a mercury-manganese star. The star is tentatively catalogued as an Alpha2 [Canum Venaticorum variable], with brightness variations of just 0.01 magnitudes.
The secondary, component B, was first separated via interferometry in 2004. It is an A-type main sequence star of class A8V. The magnitude difference between the two components is 2.64.