DI Herculis

DI Herculis is an Algol-type binary star in the constellation of Hercules. The system has an apparent magnitude of about +8.5 and consists of two young blue stars of spectral type B5 and B4. It is about two thousand light years from Earth.

The orbit of the stars around their mutual centre of gravity is very elliptical, with an eccentricity of 0.49 and a semi-major axis of 0.496 astronomical units; stellar masses of 5.15 and 4.52 solar masses lead to a theoretical precession of 4.27 degrees per century (1.93 degrees from classical effects and 2.34 degrees from general relativistic effects); but the observed precession original measure is 1.04 degrees per century and the most recent is 1.39 degrees per century.

This observation has led to extensive studies of the bright binary system in the last thirty years; solutions discussed included

After observations of the Rossiter–McLaughlin effect in 2009, it emerged that the rotation axes of the two stars lay roughly in the orbital plane of the system. When this is taken account in calculating the rate of precession, the difference between expected and observed precession disappears; so DI Hercules is no longer a test case for a possible falsification of general relativity. However, a more recent research article shows that the 2009 study leaves many questions unanswered regarding the solution for the axes. For example, orbital effects caused by the tilting of the axes have not been observed; also, the stars' rotation axes themselves may also be precessing.