Photometric elements, apsidal motion, and the third body in the eclipsing binary V974 Cyg

We have derived the first photoelectric light curve of the eclipsing binary V974 Cyg from our own photoelectric observations. Analysis of the light curve has yielded the system’s photometric elements ( r 1 ≈ r 2 = 0.1192, e = 0.058, L 1 ≈ L 2 = 0.486, and L 3 = 0.028) and absolute parameters ( M 1 ≈ M 2 = 2.2 M ⊙ , T eff ,1 ≈ T eff ,2 = 9500 K, a = 15.0 R ⊙ , distance d = 1.29 kpc, age log t = 8.0, t/tMS = 0.11). We have detected apsidal motion with the period U obs = (1140 ± 170) yrs, and the presence of a third body in the system. The orbital parameters derived for the third body are P 3 = 26.5 yrs, e 3 = 0.78, and a 3 sin i 3 = 1.5 AU; and the lower limit for its mass is M 3 > 0.58 M ⊙ . The observed apsidal-motion rate is higher than is expected theoretically by a factor of 1.5. The axial rotation of the system’s components is not yet synchronized with the orbital motion, probably because V974 Cyg is relatively young and detached.