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 \pm 0.032 M _ { J } , a radius of 0.944 \pm 0.028 R _ { J } , and an orbital period of 3.7998 d. The star has V = 13.505 \pm 0.060 , a mass of 0.645 \pm 0.020 M _ { \sun } , a radius of 0.617 \pm 0.013 R _ { \sun } , an effective temperature of T _ { eff \star } = 4390 \pm 50 , and a subsolar metallicity of [ Fe / H ] = -0.127 \pm 0.080 .
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 lowest-mass 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 .