K2-280 b-a low density warm sub-Saturn around a mildly evolved star

We present an independent discovery and detailed characterization of K2-280 b, a transiting low density warm sub-Saturn in a 19.9-d moderately eccentric orbit (e = 0.35 -0.04+0.05) from K2 campaign 7. A joint analysis of high precision HARPS, HARPS-N, and FIES radial velocity measurements and K2 photometric data indicates that K2-280 b has a radius of Rb = 7.50 ± 0.44 R⊕ and a mass of Mb = 37.1 ± 5.6 M⊕, yielding a mean density of ρb = 0.48 -0.10+0.13 g cm-3. The host star is a mildly evolved G7 star with an effective temperature of Teff = 5500 ± 100 K, a surface gravity of log g = 4.21 ± 0.05 (cgs), and an iron abundance of [Fe/H] = 0.33 ± 0.08 dex, and with an inferred mass of M∗ = 1.03 ± 0.03 M⊙ and a radius of R∗ = 1.28 ± 0.07 R⊙. We discuss the importance of K2-280 b for testing formation scenarios of sub-Saturn planets and the current sample of this intriguing group of planets that are absent in the Solar system.