Letter

Nature 461, 373-376 (17 September 2009) | doi:10.1038/nature08408; Received 12 June 2009; Accepted 14 August 2009

Misaligned spin and orbital axes cause the anomalous precession of DI Herculis

Simon Albrecht1,2, Sabine Reffert3, Ignas A. G. Snellen1 & Joshua N. Winn2

  1. Leiden Observatory, Leiden University, Postbus 9513, 2300 RA Leiden, The Netherlands
  2. Department of Physics, and Kavli Institute for Astrophysics and Space Research, Massachusetts Institute of Technology, Cambridge, Massachusetts 02139, USA
  3. Zentrum für Astronomie Heidelberg, Landessternwarte, Königstuhl 12, 69117 Heidelberg, Germany

Correspondence to: Simon Albrecht1,2 Correspondence and requests for materials should be addressed to S.A. (Email: albrecht@space.mit.edu).

The orbits of binary stars precess as a result of general relativistic effects, forces arising from the asphericity of the stars, and forces from any additional stars or planets in the system. For most binaries, the theoretical and observed precession rates are in agreement1. One system, however—DI Herculis—has resisted explanation for 30 years2, 3, 4. 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 relativity5. Among the contemporary explanations are the existence of a circumbinary planet6 and a large tilt of the stellar spin axes with respect to the orbit7, 8. 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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