EPIC 218916923 b: a low-mass warm Jupiter on a 29-day orbit transiting an active K0 V star

We announce the discovery of EPIC218916923 b, a transiting warm-Jupiter ($T_mathrm{eq}$=555$pm$11 K) on a 29-day orbit around an active (log$R^prime_mathrm{HK}$ = $-$4.49 $pm$ 0.03) K0 V star in K2 Campaign 7. We derive the systemu0027s parameters by combining the K2 photometry with ground-based follow-up observations. With a mass of $M_mathrm{p}=0.381 pm 0.045 M_{rm Jup}$ and radius of $R_mathrm{p}= 0.812 pm 0.030 R_{rm Jup}$, EPIC218916923 b is one of the transiting warm Jupiters with the lowest mass known to date. The planetary mean density $rho_mathrm{p} = 0.88 pm 0.14 {rm g,cm^{-3}}$ can be explained with a core of $sim 50 M_oplus$. Given the brightness of the host star ($V$= 11.653 mag), the relatively short transit duration ($sim$5 hours), and the expected amplitude of the Rossiter-McLaughlin effect ($sim$25 ms), EPIC218916923 is an ideal target to measure the spin-orbit angle of a planetary system hosting a warm Jupiter.