We present photometric observations in B and V as well as spectroscopic observations of the detached , eccentric 6.6-day double-lined eclipsing binary GG Ori , a member of the Orion OB1 association . Absolute dimensions of the components , which are virtually identical , are determined to high accuracy ( better than 1 % in the masses and better than 2 % in the radii ) for the purpose of testing various aspects of theoretical modeling . We obtain M _ { A } = 2.342 \pm 0.016 M _ { \sun } and R _ { A } = 1.852 \pm 0.025 R _ { \sun } for the primary , and M _ { B } = 2.338 \pm 0.017 M _ { \sun } and R _ { B } = 1.830 \pm 0.025 R _ { \sun } for the secondary . The effective temperature of both stars is 9950 \pm 200 K , corresponding to a spectral type of B9.5 . GG Ori is very close to the ZAMS , and comparison with current stellar evolution models gives ages of 65-82 Myr or 7.7 Myr depending on whether the system is considered to be burning hydrogen on the main sequence or still in the final stages of pre-main sequence contraction . Good agreement is found in both scenarios for a composition close to solar . We have detected apsidal motion in the binary at a rate of \dot { \omega } = 0.00061 \pm 0.00025 deg cycle ^ { -1 } , corresponding to an apsidal period of U = 10700 \pm 4500 yr. A substantial fraction of this ( \sim 70 % ) is due to the contribution from General Relativity , and our measurement is entirely consistent with theory . The eccentric orbit of GG Ori is well explained by tidal evolution models , but both theory and our measurements of the rotational velocity of the components are as yet inconclusive as to whether the stars are synchronized with the orbital motion .