XO-7 b: A transiting hot Jupiter with a massive companion on a wide orbit

Abstract: Transiting planets orbiting bright stars are the most favorable targets for follow-up and characterization. We report the discovery of the transiting hot Jupiter XO-7 b and of a second, massive companion on a wide orbit around a circumpolar, bright, and metal rich G0 dwarf (V = 10.52, $T_{\rm eff} = 6250 \pm 100 \; \rm K$, $\rm[Fe/H] = 0.432 \pm 0.057 \; \rm dex$). We conducted photometric and radial velocity follow-up with a team of amateur and professional astronomers. XO-7 b has a period of $ 2.8641424 \pm 0.0000043$ days, a mass of $0.709 \pm 0.034 \; \rm M_{\rm J}$, a radius of $1.373 \pm 0.026 \; \rm R_{\rm J}$, a density of $0.340 \pm 0.027 \; \rm g \, {cm}^{-3}$, and an equilibrium temperature of $1743 \pm 23 \; \rm K$. Its large atmospheric scale height and the brightness of the host star make it well suited to atmospheric characterization. The wide orbit companion is detected as a linear trend in radial velocities with an amplitude of $\sim100 \; \rm m \, {s}^{-1}$ over two years, yielding a minimum mass of $4 \; \rm M_{\rm J}$; it could be a planet, a brown dwarf, or a low mass star. The hot Jupiter orbital parameters and the presence of the wide orbit companion point towards a high eccentricity migration for the hot Jupiter. Overall, this system will be valuable to understand the atmospheric properties and migration mechanisms of hot Jupiters and will help constrain the formation and evolution models of gas giant exoplanets.