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We present a high-precision I-band light curve for the Wolf-Rayet binary WR20a, obtained as a sub-project of the Optical Gravitational Lensing Experiment.Rauw et al. have recently presented spectroscopy for this system, stronglysuggesting extremely large minimum masses of 70.7 +/- 4.0 Mo and 68.8 +/- 3.8Mo for the component stars of the system, with the exact values dependingstrongly on the period of the system. We detect deep eclipses of about 0.4 magin the light curve of WR 20a, confirming and refining the suspected period ofP=3.686 days and deriving an inclination angle of i=74.5 +/- 2 deg. Using thesephotometric data and the radial velocity data of Rauw et al., we derive themasses for the two components of WR 20a to be 83.0 +/- 5.0 Mo and 82.0 +/- 5.0Mo. Therefore, WR 20a is confirmed to consist of two extremely massive starsand to be the most massive binary known with an accurate mass determination.Comment: 13 pages, 2 figures, 3 tables, accepted for publication in ApJ

WR 20a Is an Eclipsing Binary: Accurate Determination of Parameters for an Extremely Massive Wolf-Rayet System