Radial velocities of HD 118203

We present the discovery of a long-period giant planet, HD 118203 c (m_c=11.79_-0.63_^+0.69^M_J_, a_c_=6.28_-0.11_^+0.10^AU) exterior to a close-in eccentric hot Jupiter HD 118203 b (P_b_=6.135d, m_b_=2.14+/-0.12M_J_, r_b_=1.14+/-0.029R_J_, e_b_=0.31+/-0.007) based on 20yr radial velocities (RVs). Using Rossiter-McLaughlin (RM) observations from the Keck Planet Finder, we measured a low sky-projected spin-orbit angle {lambda}_b_=-11.7{deg}_-10.0_^+7.6^ for HD 118203 b and detected stellar oscillations in the host star, confirming its evolved status.

Combining the RM observation with the stellar inclination measurement, we constrained the true spin-orbit angle of HD 118203 b as {Psi}_b_<33.5{deg} (2{sigma}), indicating the orbit normal of the hot Jupiter nearly aligned with the stellar spin axis. Furthermore, by combining RVs and Hipparcos-Gaia astrometric acceleration, we constrained the line-of-sight mutual inclination between the hot Jupiter and the outer planet to be 9.8{deg}_-9.3_^+16.2^ at the 2{sigma} level.