Epsilon Aurigae (ε Aur) is a visually bright, eclipsing binary star system with a period of 27.1 years. The cause of each 18-month-long eclipse has been a subject of controversy for nearly 190 years1 because the companion has hitherto been undetectable. The orbital elements imply that the opaque object has roughly the same mass as the visible component, which for much of the last century was thought to be an F-type supergiant star with a mass of 15M (M, mass of the Sun). The high mass-to-luminosity ratio of the hidden object was originally explained by supposing it to be a hyperextended infrared star2 or, later, a black hole3 with an accretion disk, although the preferred interpretation was as a disk of opaque material4,5 at a temperature of 500 K, tilted to the line of sight6,7 and with a central opening8. Recent work implies that the system consists of a low-mass (2.2M–3.3M) visible F-type star, with a disk at 550 K that enshrouds a single B5V-type star9. Here we report interferometric images that show the eclipsing body moving in front of the F star. The body is an opaque disk and appears tilted as predicted7. Adopting a mass of 5.9M for the B star, we derive a mass of (3.6 ± 0.7)M for the F star. The disk mass is dynamically negligible; we estimate it to contain 0.07M (M, mass of the Earth) if it consists purely of dust.

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We are grateful to the firefighters who defended Mount Wilson Observatory from the Station Fire, and L. Webster and the staff at Mount Wilson Observatory for facilitating our observations. We acknowledge with thanks the variable-star observations from the AAVSO International Database contributed by observers worldwide and used in this research. The CHARA Array, operated by Georgia State University, was built with funding provided by the US National Science Foundation (NSF), Georgia State University, the W. M. Keck Foundation and the David and Lucile Packard Foundation. CHARA is operated under continuing support from the NSF. This research is supported by the NSF as well as by funding from the office of the Dean of the College of Arts and Science at Georgia State University. J.D.M. acknowledges funding from the University of Michigan and the NSF. MIRC was supported by funds from the NSF. B.K. and R.S. thank J. Hopkins for ongoing photometry and are grateful for the bequest of William Herschel Womble in support of astronomy at the University of Denver.

Author Contributions R.S. originally proposed this research task, facilitated observations and arranged for concurrent observations at other research facilities. Raw data from MIRC was reduced by J.D.M. using calibrated diameters from X.C. and literature sources. Image reconstruction and modelling was performed by F.B., J.D.M. and B.K. Determination of the F star’s translational speed was done by G.S. and B.K. Observations were planned by R.S., G.S., B.K. and M.Z. and were facilitated by J.S., E.P., L.S., N. Thureau, N. Turner, and X.C. The data necessary for this publication was collected by M.Z., B.K., G.S., C.F., P.J.S.-G., R.S. and F.B. Discussion of historical models and their implication to observations was conducted by S.M.C., B.K. and R.S. Administrative oversight and access to CHARA was provided by H.M. and T.t.B. All authors discussed the results and commented on the manuscript.

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  1. Department of Physics and Astronomy, University of Denver, 2112 East Wesley Avenue, Denver, Colorado 80208, USA

    • Brian Kloppenborg
    •  & Robert Stencel
  2. Department of Astronomy, University of Michigan, 500 Church Street, Ann Arbor, Michigan 48109-1090, USA

    • John D. Monnier
    • , Fabien Baron
    •  & Xiao Che
  3. Center for High Angular Resolution Astronomy, Georgia State University, PO Box 3969, Atlanta, Georgia 30302-3969, USA

    • Gail Schaefer
    • , Hal McAlister
    • , Theo ten Brummelaar
    • , Chris Farrington
    • , P. J. Sallave-Goldfinger
    • , Judit Sturmann
    • , Laszlo Sturmann
    •  & Nils Turner
  4. Jet Propulsion Laboratory, 4800 Oak Grove Drive, M.S. 169-327, Pasadena, California 91101, USA

    • Ming Zhao
  5. School of Physics and Astronomy, University of St Andrews, North Haugh, St Andrews KY16 9SS, UK

    • Ettore Pedretti
    •  & Nathalie Thureau
  6. Physics Department, California Institute of Technology, 1200 East California Avenue, Pasadena, California 91125, USA

    • Sean M. Carroll


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The authors declare no competing financial interests.

Corresponding authors

Correspondence to Brian Kloppenborg or Robert Stencel.

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