1. Department of Physics and Tuorla Observatory, University of Turku, Vaisäläntie 20, FI-21500 Piikkiö, Finland
2. Landessternwarte Heidelberg, Königstuhl, 69117 Heidelberg, Germany
3. Herschel Science Centre, European Space Astronomy Centre, European Space Agency, Villafrance del Castillo Satellite Tracking Station, PO Box 78, 28691 Villanueva de la Canada, Madrid, Spain, and INSA, Paseo del Pintor Rosales 34, 28008 Madrid, Spain
4. British Astronomical Association Variable Star Section, 67 Ellerton Road, Kingstanding, Birmingham B44 0QE, UK
5. Nordic Optical Telescope, Apartado 474, E-38700 S/C de La Palma, Spain
6. Astronomical Observatory of the Jagiellonian University, u1. Orla 171, 30-224 Cracow, Poland
7. Mt. Suhora Observatory, Pedagogical University, ul. Podchorazych 2, 30-084 Cracow, Poland
8. National Astronomical Observatories, Chinese Academy of Sciences, 20A Datun Road, Beijing 100012, China
9. Astronomical Institute, Osaka-Kyoiku University, Asahigaoka, Kashiwara, Osaka 582-8582, Japan
10. Department of Physics, Shoumen University, 9700 Shoumen, Bulgaria
11. Warkauden Kassiopeia ry, Härkämäentie 88, 79480 Kangaslampi, Finland
12. Department of Astrophysics, Astronomy and Mechanics, Faculty of Physics, University of Athens, Panepistimiopolis, GR-15784 Zografos, Athens, Greece
13. Canakkale Onsekiz Mart University, Faculty of Physics, TR-17020 Canakkale, Turkey

Correspondence to: M. J. Valtonen¹ Correspondence and requests for materials should be addressed to M.J.V. (Email: mvaltonen2001@yahoo.com).

Tests of Einstein's general theory of relativity have mostly been carried out in weak gravitational fields where the space-time curvature effects are first-order deviations from Newton's theory^{1, 2, 3, 4, 5, 6}. Binary pulsars⁴ provide a means of probing the strong gravitational field around a neutron star, but strong-field effects may be best tested in systems containing black holes^{7, 8}. Here we report such a test in a close binary system of two candidate black holes in the quasar OJ 287. This quasar shows quasi-periodic optical outbursts at 12-year intervals, with two outburst peaks per interval^{9, 10}. The latest outburst occurred in September 2007, within a day of the time predicted by the binary black-hole model and general relativity¹¹. The observations confirm the binary nature of the system and also provide evidence for the loss of orbital energy in agreement (within 10 per cent) with the emission of gravitational waves from the system¹². In the absence of gravitational wave emission the outburst would have happened 20 days later¹³.

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