Photometric elements, apsidal motion, and a third body in the eclipsing binary HP Aur

Highly accurate W BV R photometric measurements of the eclipsing binary HP Aur were performed in 2002–2003 with the 48-cm AZT-14 reflector at the Tien-Shan High-Altitude Observatory to determine the rate of apsidal motion. A consistent system of physical and geometrical parameters of the components and the binary as a whole has been constructed for the first time by analyzing these new measurements together with other published data: we determined their radii (R 1 = 1.05R ⊙, R 2 = 0.82R ⊙) and luminosities (L 1 = 1.10L ⊙, L 2 = 0.46L ⊙), spectral types (G2V + G8V) and surface gravities (log g 1 = 4.38, log g 2 = 4.51), age (t = 9.5 × 109 yr), and the distance to the binary (d = 197 pc). We detected an ultraviolet excess in the spectra of both components, $$\Delta (W - B) \simeq - 0\mathop .\limits^m 25$$ , that is probably attributable to a metal deficiency in the atmospheres of these stars. In this system of two solar-type stars, we found a third body with the mass M 3 sin i 3 3 = 0.17M ⊙ that revolved with the period P3 = 13.7 yr around the eclipsing binary in a highly eccentric elliptical orbit: e 3 = 0.70 and A 3 sin i 3 ≃ 7 AU. The orbit of the eclipsing binary itself was shown to be also elliptical, but with a low eccentricity (e = 0.0025(5)), while apsidal motion with a period U obs > 80 yr was observed at a theoretically expected period U th ≈ 92 yr. At least 20 to 30 more years of photoelectric measurements of this star will be required to reliably determine U obs.

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