DI Herculis is an Algol-type binary star in the constellation of Hercules. The system has an ensemble 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 0.2 and a semi-major axis of 0.496 astronomical units; stellar masses of 4.52 and 5.15 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
- new theories of gravitation such as MOND
- tidal forces (perhaps due to unusual internal structure in the stars) leading to a circularisation of the elliptical orbit
- a third body in the system 
- presence of a circumstellar cloud between the two components
- unusual rotation axes of the stars
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, "The Eclipsing Binary DI Herculis: One Mystery Solved, But Another Takes Its Place", shows that the 2009 study leaves many questions unanswered regarding DI Herculis axes solution. This research states that DI Hercules precession has changed over the past 30 years.
- DI Herculis - Ein ungewöhnlich exzentrisches Algolsystem; Astronomische Nachrichten, volume 265, p.101, 1938
- S. Albrecht, S. Reffert, I. Snellen (2009). "Misaligned spin and orbital axes cause the anomalous precession of DI Herculis" 461. Nature. pp. 373–376. arXiv:0909.2861v1. Bibcode:2009Natur.461..373A. doi:10.1038/nature08408.
- A. Claret, G. Torres, M. Wolf (2010). "DI Herculis as a test of internal stellar structure and general relativity" 515. Astronomy & Astrophysics. arXiv:1002.2949. Bibcode:2010A&A...515A...4C. doi:10.1051/0004-6361/200913942.
- A. Dariush, N. Riazi (2003). "A New Apsidal Motion Study of DI Her" 283. Astrophysics and Space Science. pp. 253–261. Bibcode:2003Ap&SS.283..257D. doi:10.1023/A:1021380815734.
- UBVR photometry of DI Herculis
- Hsuan, Keith; Mardling, Rosemary A (2006). Astrophysics and Space Science 304: 243–246. Bibcode:2006Ap&SS.304..243H. doi:10.1007/s10509-006-9121-0.
- Khodykon, S A. "Evidence for a Third Body in the Eclipsing Binary DI Herculis". Information Bulletin on Variable Stars (5788).