HAT-P-54b: A hot jupiter transiting A 0.64 M_o star in field 0 of the K2 mission

We report the discovery of HAT-P-54b, a planet transiting a late K dwarf star in field 0 of the NASA K2 mission. We combine ground-based photometric light curves with radial velocity measurements to determine the physical parameters of the system. HAT-P-54b has a mass of 0.760 ± 0.032 MJ, a radius of 0.944 ± 0.028 RJ, and an orbital period of 3.7998 days. The star has V = 13.505 ± 0.060, a mass of 0.645 ± 0.020 ⊙ M , a radius of 0.617 ± 0.013 ⊙ R , an effective temperature of Teff∗ = 4390 ± 50, and a subsolar metallicity of [Fe H] = -0.127 ± 0.080. We also detect a periodic signal with P = 15.6 days and 5.6 mmag amplitude in the light curve, which we interpret as due to the rotation of the star. HAT-P-54b has a radius that is smaller than 92% of the known transiting planets with masses greater than that of Saturn, while HAT-P-54 is one of the lowestmass stars known to host a hot Jupiter. Follow-up high-precision photometric observations by the K2 mission promise to make this a well-studied planetary system.