  We derive absolute dimensions of the early B-type detached eclipsing binary
V453 Cygni (B0.4 IV + B0.7 IV, P=3.89d), a member of the open cluster NGC 6871.
  From the analysis of new, high-resolution, spectroscopy and the UBV light
curves of Cohen (1974) we find the masses to be 14.36 +/- 0.20 and 11.11 +/-
0.13 Msun, the radii to be 8.55 +/- 0.06 and 5.49 +/- 0.06 Rsun, and the
effective temperatures to be 26600 +/- 500 and 25 500 +/- 800 K for the primary
and secondary stars, respectively. The surface gravities of 3.731 +/- 0.012 and
4.005 +/- 0.015 indicate that V453 Cyg is reaching the end of its main sequence
lifetime. We have determined the apsidal motion period of the system to be 66.4
+/- 1.8 yr using the technique of Lacy (1992) extended to include spectroscopic
data as well as times of minimum light, giving a density concentration
coefficient of log(k_2) = -2.226 +/- 0.024. Contaminating (third) light has
been detected for the first time in the light curve of V453 Cyg; previous
analyses without this effect systematically underestimate the ratio of the
radii of the two stars. The absolute dimensions of the system have been
compared to the stellar evolution models of the Granada, Geneva, Padova and
Cambridge groups. All model sets fit the data on V453 Cyg for solar helium and
metal abundances and an age of 10.0 +/- 0.2 Myr. The Granada models also agree
fully with the observed log(k_2) once general relativistic effects have been
accounted for. The Cambridge models with convective core overshooting fit V453
Cyg better than those without. Given this success of the theoretical
predictions, we briefly discuss which eclipsing binaries should be studied in
order to further challenge the models.
