  The KESPRINT consortium identified K2-216 as a planetary candidate host star
in the K2 space mission Campaign 8 field with a transiting super-Earth. The
planet has recently been validated as well. Our aim was to confirm the
detection and derive the main physical characteristics of K2-216b, including
the mass. We performed a series of follow-up observations: high resolution
imaging with the FastCam camera at the TCS, the Infrared Camera and
Spectrograph at Subaru, and high resolution spectroscopy with HARPS (ESO, La
Silla), HARPS-N (TNG), and FIES (NOT). The stellar spectra were analyzed with
the SpecMatch-Emp and SME codes to derive the stellar fundamental properties.
We analyzed the K2 light curve with the Pyaneti software. The radial-velocity
measurements were modelled with both a Gaussian process (GP) regression and the
floating chunk offset (FCO) technique to simultaneously model the planetary
signal and correlated noise associated with stellar activity. Imaging confirms
that K2-216 is a single star. Our analysis discloses that the star is a
moderately active K5V star of mass 0.70+/-0.03 Msun and radius 0.72+/-0.03
Rsun. Planet b is found to have a radius of 1.75+0.17-0.10 Rearth and a
2.17-day orbit in agreement with previous results. We find consistent results
for the planet mass from both models: 7.4+/-2.2 Mearth from the GP regression,
and 8.0+/-1.6 Mearth from the FCO technique, which implies that this planet is
a super-Earth. The planet parameters put planet b in the middle of, or just
below, the gap of the radius distribution of small planets. The density is
consistent with a rocky composition of primarily iron and magnesium silicate.
In agreement with theoretical predictions, we find that the planet is a remnant
core, stripped of its atmosphere, and is one of the largest planets found that
has lost its atmosphere.
