K2-141 b A 5-M-circle plus super-Earth transiting a K7V star every 6.7 h

We report on the discovery of K2-141 b (EPIC 246393474 b), an ultra-short-period super-Earth on a 6.7 h orbit transiting an active K7V star based on data from K2 campaign 12. We confirmed the planet's existence and measured its mass with a series of follow-up observations: seeing-limited MuSCAT imaging, NESSI high-resolution speckle observations, and FIES and HARPS high-precision radial-velocity monitoring. K2-141 b has a mass of 5.31 +/- 0.46 M-circle plus and radius of 1.54(-0.09)(+0.10), yielding a mean density of 8.00(-1.45)(+1.83) g cm(-3) and suggesting a rocky-iron composition. Models indicate that iron cannot exceed similar to 70% of the total mass. With an orbital period of only 6.7 h, K2-141 b is the shortest-period planet known to date with a precisely determined mass.