We report the discovery of KELT-18b , a transiting hot Jupiter in a 2.87d orbit around the bright ( V = 10.1 ) , hot , F4V star BD+60 1538 ( TYC 3865-1173-1 ) .
We present follow-up photometry , spectroscopy , and adaptive optics imaging that allow a detailed characterization of the system .
Our preferred model fits yield a host stellar temperature of 6670 \pm 120 K and a mass of 1.524 _ { -0.068 } ^ { +0.069 } M _ { \Sun } , situating it as one of only a handful of known transiting planets with hosts that are as hot , massive , and bright .
The planet has a mass of 1.18 \pm 0.11 M _ { J } , a radius of 1.570 _ { -0.036 } ^ { +0.042 } R _ { J } , and a density of 0.377 \pm 0.040 ~ { } g~ { } cm ^ { -3 } , making it one of the most inflated planets known around a hot star .
We argue that KELT-18b ’ s high temperature and low surface gravity , which yield an estimated \sim 600 km atmospheric scale height , combined with its hot , bright host make it an excellent candidate for observations aimed at atmospheric characterization .
We also present evidence for a bound stellar companion at a projected separation of \sim 1100 AU , and speculate that it may have contributed to the strong misalignment we suspect between KELT-18 ’ s spin axis and its planet ’ s orbital axis .
The inferior conjunction time is 2457542.524998 \pm 0.000416 ( BJD _ { TDB } ) and the orbital period is 2.8717510 \pm 0.0000029 days .
We encourage Rossiter-McLaughlin measurements in the near future to confirm the suspected spin-orbit misalignment of this system .