Brewing another binary in the Teapot: Nandou=Phi Sagittarii

The iconic Teapot asterism in Sagittarius is particularly rich in intermediate-mass stars in multiple systems. Here we combine VLTI/GRAVITY, archival spectroscopic observations, past radial velocity measurements and photometry to solve for the orbital and stellar parameters of the binary Nandou=Phi Sagittarii. It has a period $P=238.844\pm0.011$ days, a semi-major axis $a=1.32\pm0.10\text{ au}$ and eccentricity $e=0.418\pm0.004$. The precision in the dynamical masses $M_A=3.8\pm0.9 M_{\odot}$ and $M_B=1.6\pm0.4 M_{\odot}$ is currently limited by the parallax $p=12.5\pm1.0 \text{ mas}$. We find that the original Hipparcos 1997 Intermediate Astrometric Data (IAD) are consistent with the expected photocenter semi-major axis $a_{\mathrm{phot}} \sim 5 \text{ mas}$, while the IAD from the Hipparcos 2007 new reduction yield a too small $a_{\mathrm{phot}} = 1.7\pm0.3 \text{ mas}$, confirming the suspicion that a too aggressive modeling of the satellite attitude can lead to overfitting that suppresses the orbital motion. Matching the primary's temperature $T_{\mathrm{eff,A}} = 12000\pm250 \text{ K}$ and radius $R_{\mathrm{A}} = 5.1\pm 0.4 R_{\odot}$ to stellar evolutionary models implies that it is a $M_A \simeq 4 M_{\odot}$ star near the end of the main-sequence. Nandou will eventually most likely become a symbiotic star.