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We present photometric observations in B and V as well as spectroscopicobservations of the detached, eccentric 6.6-day double-lined eclipsing binaryGG Ori, a member of the Orion OB1 association. Absolute dimensions of thecomponents, which are virtually identical, are determined to high accuracy(better than 1% in the masses and better than 2% in the radii) for the purposeof testing various aspects of theoretical modeling. We obtain M(A) = 2.342 +/-0.016 solar masses and R(A) = 1.852 +/- 0.025 solar radii for the primary, andM(B) = 2.338 +/- 0.017 solar masses and R(B) = 1.830 +/- 0.025 solar radii forthe secondary. The effective temperature of both stars is 9950 +/- 200 K,corresponding to a spectral type of B9.5. GG Ori is very close to the ZAMS, andcomparison with current stellar evolution models gives ages of 65-82 Myr or 7.7Myr depending on whether the system is considered to be burning hydrogen on themain sequence or still in the final stages of pre-main sequence contraction. Wehave detected apsidal motion in the binary at a rate of dw/dt = 0.00061 +/-0.00025 degrees per cycle, corresponding to an apsidal period of U = 10700 +/-4500 yr. A substantial fraction of this (approximately 70%) is due to thecontribution from General Relativity.Comment: To appear in The Astronomical Journal, December 200

Absolute Dimensions of the Unevolved B-Type Eclipsing Binary GG Orionis