HAT-P-57b is a giant planet in a close-in orbit around a rapidly rotating A8V star. A-type stars are more massive and much more luminous than F, G and K-type stars. To date, only a handful of planets have been found around A-type stars. HAT-P-57b was detectable because it transits its host star every 2.465 days, which is also the orbital period of the planet around its host star. Such a short orbital period implies HAT-P-57b is in an extremely close-in orbit, only about 5 stellar radii from the surface of its host star. HAT-P-57b is estimated to have an equilibrium temperature of roughly 2200 K.
The host star of is estimated to have 1.47 ± 0.12 times the mass, 1.500 ± 0.050 times the radius and 6.4 ± 1.1 times the luminosity of the Sun. With an equatorial rotation velocity of at least 102 km/s, the host star of HAT-P-57b has the highest rotation velocity of any star currently known to host a transiting planet. The next most rapidly rotating stars to host transiting planets are KOI-89 with a rotation velocity of at least 90 km/s and WASP-33 with a rotation velocity of at least 86 km/s. Finally, the mass of HAT-P-57b is estimated to be no more than 1.85 times the mass of Jupiter.
Transit light curve indicating the presence of HAT-P-57b. Hartman et al. (2015)
Hartman et al. (2015), “HAT-P-57b: A Short-Period Giant Planet Transiting A Bright Rapidly Rotating A8V Star Confirmed Via Doppler Tomography”, arXiv:1510.08839 [astro-ph.EP]