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The Kepler extended mission, also known as K2, has provided the communitywith a wealth of planetary candidates that orbit stars typically much brighterthan the targets of the original mission. These planet candidates are suitablefor further spectroscopic follow-up and precise mass determinations, leadingultimately to the construction of empirical mass-radius diagrams. Particularlyinteresting is to constrain the properties of planets between the Earth andNeptune in size, the most abundant type of planets orbiting Sun-like stars withperiods less than a few years. Among many other K2 candidates, we discovered amulti-planetary system around EPIC246471491, with four planets ranging in sizefrom twice the size of Earth, to nearly the size of Neptune. We measure themass of the planets of the EPIC246471491 system by means of precise radialvelocity measurements using the CARMENES spectrograph and the HARPS-Nspectrograph. With our data we are able to determine the mass of the two innerplanets of the system with a precision better than 15%, and place upper limitson the masses of the two outer planets. We find that EPIC246471491b has a massof 9.68 Me, and a radius of 2.59 Re, yielding a mean density of 3.07 g/cm3,while EPIC246471491c has a mass of 15.68 Me, radius of 3.53 Re, and a meandensity of 19.5 g/cm3. For EPIC246471491d (R=2.48Re) and EPIC246471491e(R=1.95Re) the upper limits for the masses are 6.5 and 10.7 Me, respectively.The system is thus composed of a nearly Neptune-twin planet (in mass andradius), two sub-Neptunes with very different densities and presumably bulkcomposition, and a fourth planet in the outermost orbit that resides right inthe middle of the super-Earth/sub-Neptune radius gap. Future comparativeplanetology studies of this system can provide useful insights into planetaryformation, and also a good test of atmospheric escape and evolution theories.

Detection and Doppler monitoring of EPIC 246471491, a system of four transiting planets smaller than Neptune