This image is courtesy of Sky and Telescope magazine.
DI Herculis consists of two young blue stars of spectral type B5 and B4. It is about two thousand light years from Earth
Since the early 1980's astronomers have puzzeled over the fact that the apsidal motion of this binary star system is about 1/3 the theoretical prediction expected from a combination of general relativity and classical tidal-rotational effects. But in a paper published in the Astrophysical Journal ( Ap.J volume 375, p. 314) physicists Khaliullin, Khodykin and Zakharov from the Moscow University have shown that the discrepancy in the apsidal motion could be explained in full by the action of a third body in the system. The third star would be in an orbit with a maximum distance of 0.02" from the binary and have a luminosity equal to 3 percent of the Sun's.
Then in 2009, astronomer Simon Albrecht and his colleagues showed that all you need for this binary system is for the rotation axis of the stars to be misaligned with the orbital axis of the system. As they note in the abstract to the Nature article:
The observed precession rate is a factor of four slower than the theoretical rate, a disagreement that once was interpreted as evidence for a failure of general relativity. Among the contemporary explanations are the existence of a circumbinary planet and a large tilt of the stellar spin axes with respect to the orbit. Here we report that both stars of DI Herculis rotate with their spin axes nearly perpendicular to the orbital axis (contrary to the usual assumption for close binary stars). The rotationally induced stellar oblateness causes precession in the direction opposite to that of relativistic precession, thereby reconciling the theoretical and observed rates.
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