Eclipsing binaries as astrophysical laboratories:: Internal structure, core convection, and evolution of the B-star components of V380 Cygni

New photometric solutions have been carried out on the important eccentric eclipsing system V380 Cygni (B1.5 II-III + B2 V) from UBV differential photoelectric photometry obtained by us. The photometric elements obtained from the analysis of the light curves have been combined with the spectroscopic solution recently published by Popper & Guinan and have led to the physical properties of the system components. The effective temperature of the stars has been determined by fitting IUE UV spectrophotometry to Kurucz model atmospheres and compared with other determinations from broadband and intermediate-band standard photometry. The values of mass, absolute radius, and effective temperature for the primary and secondary stars are 11.1 +/- 0.5 M., 14.7 +/- 0.2 R., 21,350 +/- 400 K, and 6.95 +/- 0.25 M., 3.74 +/- 0.07 R., 20,500 +/- 500 K, respectively. In addition, a redetermination of the system's apsidal motion rate has been done from the analysis of 12 eclipse timings obtained from 1923 to 1995. The apsidal motion study yields the internal mass distribution of the more luminous component. Using stellar structure and evolutionary models with modern input physics, tests on the extent of convection in the core of the more massive B1.5 II-III star of the system have been carried out. Both the analysis of the log g-log T-eff diagram and the apsidal motion study indicate a. star with a larger convective core, and thus more centrally condensed, than currently assumed. This has been quantified in form of an overshooting parameter with a value of alpha (ov) approximate to 0.6 +/- 0.1. Finally, the tidal evolution of the system (synchronization and circularization times) has also been studied.