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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We are grateful to Á. Giménez, E. Guinan and T. Mazeh for bringing DI Herculis to our attention. We thank J. Lub, R. Tubbs, C. Hopman, Y. Levin and D. Fabrycky for discussions about double stars and their properties. We also thank H. Reckman for essential help during one observing run. We are grateful to the Sophie team for building the spectrograph and reduction pipeline. This research has made use of the SIMBAD database, operated at CDS, Strasbourg, France, and the Vienna Atomic Line database (VALD) located at http://ams.astro.univie.ac.at/vald/. S.A. acknowledges support during part of this project by a Rubicon fellowship from the Netherlands Organisation for Scientific Research (NWO). J.N.W. acknowledges support from a NASA Origins grant (NNX09AD36G). S.A. and S.R. acknowledge funding from the Optical Infrared Coordination network (OPTICON).
Author Contributions S.A. participated in the development of the concept of this research and the analysis code, participated in the observations, the analysis and interpretation of the data and writing the manuscript. S.R. participated in the development of the concept of this research and the analysis code and the observations. I.A.G.S. participated in the analysis and interpretation of the data and in writing the manuscript. J.N.W. participated in the analysis and interpretation of the data and in writing the manuscript.
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Albrecht, S., Reffert, S., Snellen, I. et al. Misaligned spin and orbital axes cause the anomalous precession of DI Herculis. Nature 461, 373–376 (2009). https://doi.org/10.1038/nature08408