Photometric elements and apsidal motion of the eclipsing binary NSV 18773

The photometric elements of the eclipsing binary NSV 18773 (HD 99898) have been determined for the first time by analyzing its V- and I-band light curves from the ASAS-2 and ASAS-3 catalogs. Based on these elements and using other published spectroscopic and photometric data, we constructed a consistent system of geometrical and physical parameters for the system that consists of two stars (M-1 = 20M(circle dot), Sp(1) = B0V, R-1 = 5.0R(circle dot) and M-2 = 14M(circle dot), Sp(2) = B1V, R-2 = 6.5R(circle dot)) in elliptical orbits (P = 5(d).049, e = 0.365, a = 40.1R(circle dot)). The distance to the system is d = 3.3 kpc, the interstellar extinction is A(V) = 2(m).0, and the age is t = 2.8 x 10(6) yr. NSV 18773 is a visual binary with components V-A = 9(m). 9 and V-B = 10(m).3 separated by 0".8. The third light (L-3 = 0.61) that we found by analyzing the light curves shows that the eclipsing binary is the system's fainter component B. We confirmed the rapid apsidal motion of the star detected by Otero and Wils (2006) and refined its observed period: U-obs = 150 +/- 6 yr. Our photometric elements and physical parameters allowed the apsidal parameter (k) over bar (obs)(2) = 0.0135(14), which reflects the density distribution along the radii of the component stars, to be determined. Within the error limits, the derived parameter agrees with its theoretically expected value, (k) over bar (th)(2) = 0.0119( 8), from current evolutionary models of stars of the corresponding masses and ages.