TESS-Keck Survey. IX. Masses of Three Sub-Neptunes Orbiting HD 191939 and the Discovery of a Warm Jovian plus a Distant Substellar Companion

Exoplanet systems with multiple transiting planets are natural laboratories for testing planetary astrophysics. One such system is HD 191939 (TOI-1339), a bright (V=9) and Sun-like (G9V) star, which TESS found to host three (b, c, d). The have periods of 9, 29, 38 days each similar sizes from 3 3.4 $R_{\oplus}$. To further characterize the system, we measured radial velocity (RV) over 415 Keck/HIRES APF/Levy. We find that $M_b = 10.4 \pm 0.9 M_{\oplus}$ $M_c 7.2 1.4 M_{\oplus}$, low compared most known comparable radii. RVs yield only an upper-limit on $M_d$ (<5.8 $M_{\oplus}$ at 2$\sigma$). reveal fourth planet (e) minimum mass $0.34 0.01 M_{Jup}$ orbital period 101.4 $\pm$ 0.4 days. Despite its non-transiting geometry, secular interactions between e inner indicate coplanar ($\Delta$i < 10$^{\circ}$). identify second high (f) 95% confidence intervals $2-11 \, 1700-7200 days, based joint analysis astrometry $Gaia$ $Hipparcos$. As star hosting well-measured masses, presents many options comparative astronomy including characterization JWST.