Discovery of an inflated hot Jupiter around a slightly evolved star TOI-1789

We report here the discovery of a hot Jupiter at an orbital period of 3.208664 ± 0.000015 d around TOI-1789 (TYC 1962-00303-1, TESS_mag = 9.1) based on the TESS photometry, ground-based photometry, and high-precision radial velocity (RV) observations. The high-precision RV observations were obtained from the high-resolution spectrographs, PARAS at Physical Research Laboratory (PRL), India, and TCES at Thüringer Landessternwarte Tautenburg (TLS), Germany, and the ground-based transit observations were obtained using the 0.43-m telescope at PRL with the Bessel-R filter. The host star is a slightly evolved (log g_* = $3.943^{+0.023}_{-0.043}$), late F-type (T_eff= 5991 ± 55 K), metal-rich star ([Fe/H] = $0.373^{+0.071}_{-0.086}$ dex) with a radius of $\, R_*$ = $2.168^{+0.036}_{-0.034}$ R_⊙ located at a distance of $223.53^{+0.91}_{-0.90}$ pc. The simultaneous fitting of the multiple light curves and the RV data of TOI-1789 reveals that TOI-1789 b has a mass of $\, M_{\rm P}$ = 0.70 ± 0.16 $\, M_{\rm J}$, a radius of $\, R_{\rm P}$ = $1.44^{+0.24}_{-0.14}$ $\, R_{\rm J}$, and a bulk density of ρ_P = $0.28^{+0.14}_{-0.12}$ g cm^-3 with an orbital separation of a = $0.04882^{+0.00063}_{-0.0016}$ au. This puts TOI-1789 b in the category of inflated hot Jupiters. It is one of the few nearby evolved stars with a close-in planet. The detection of such systems will contribute to our understanding of mechanisms responsible for inflation in hot Jupiters and also provide an opportunity to understand the evolution of planets around stars leaving the main-sequence branch.