We announce the discovery of K2-139 b ( EPIC 218916923 b ) , a transiting warm-Jupiter ( T _ { \mathrm { eq } } =547 \pm 25 K ) on a 29-day orbit around an active ( log R ^ { \prime } _ { \mathrm { HK } } = - 4.46 \pm 0.06 ) K0 V star in K2 Campaign 7 .
We derive the system ’ s parameters by combining the K2 photometry with ground-based follow-up observations .
With a mass of 0.387 _ { -0.075 } ^ { +0.083 } M _ { \mathrm { J } } and radius of 0.808 _ { -0.033 } ^ { +0.034 } R _ { \mathrm { J } } , K2-139 b is one of the transiting warm Jupiters with the lowest mass known to date .
The planetary mean density of 0.91 _ { -0.20 } ^ { +0.24 } \mathrm { 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 m s ^ { -1 } ) , K2-139 is an ideal target to measure the spin-orbit angle of a planetary system hosting a warm Jupiter .