Which is the most eccentric binary known? Insights from the 2023/4 pericenter passages of Zeta Boötis and Eta Ophiuchi

There is a clear dearth of very eccentric binaries among those for which individual eccentricities can be measured. In this paper we report on observations of the two nearby, bright and very eccentric visual binaries Zeta Bo"otis ($\zeta$ Boo) and Eta Ophiuchi ($\eta$ Oph), for which VLTI/GRAVITY interferometric observations were obtained during their pericenter passages in 2023/4. Previous observations of $\zeta$ Boo suggest an eccentricity $e>0.99$ with high significance, implying that it has the highest eccentricity of any known binary. However, our interferometric measurements near periastron passage reveal that the eccentricity is actually $e=0.980450\pm0.000064$ (second highest well constrained eccentricity) with a pericenter distance $a_p=0.818\pm0.009\text{ au}$. We attribute the previous over-estimation to a degeneracy that plagues very eccentric visual binary orbital solutions. For $\eta$ Oph we find an eccentricity $e=0.93077\pm0.00013$ (compared to previous estimates of $e=0.95\pm 0.02$), a pericenter distance $a_p=2.15\pm0.10 \text{ au}$ and attribute the over-estimated dynamical mass in the previous solution to an underestimated error in the semi-major axis. In both systems full circularization is expected as the stars evolve and expand, ultimately leading to close binaries with no memory of their very eccentric past.