Absolute dimensions of the unevolved B-type eclipsing binary GG Orionis

We present photometric observations in B and V, as well as spectroscopic ob servations 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 purp ose of testing various aspects of theoretical modeling. We obtain M-A = 2.3 42 +/- 0.016 M-. and R-A = 1.852 +/- 0.025 R-. for the primary, and M-B = 2 .338 +/- 0.017 M-. and R-A = 1.830 +/- 0.025 R-. for the secondary. The eff ective temperature of both stars is 9950 +/- 200 K, corresponding to a spec tral type of B9.5. GG Ori is very close to the zero-age main sequence, 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 hydro gen 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 cl ose to solar. We have detected apsidal motion in the binary at a rate of (o mega) over dot = 0 degrees .00061 +/- 0 degrees .00025 cycle(-1), correspon ding to an apsidal period of U = 10,700 +/- 4500 yr. A substantial fraction of this (similar to 70%) is due to the contribution from general relativit y, and our measurement is entirely consistent with theory. The eccentric or bit 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 ye t inconclusive as to whether the stars are synchronized with the orbital mo tion.