K2-139 b: a low-mass warm Jupiter on a 29-d orbit transiting an active K0 V star

We announce the discovery of K2-139 b (EPIC 218916923 b), a transiting warm-Jupiter (T_eq = 547 ± 25 K) on a 29-d orbit around an active (log R^' _HK = -4.46 ± 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_J and radius of 0.808 _{ - 0.033 } ^ {+ 0.034 }R_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 } g cm^-3can be explained with a core of ∼50 M_⊕. Given the brightness of the host star (V = 11.653 mag), the relatively short transit duration (∼5 h), and the expected amplitude of the Rossiter-McLaughlin effect (∼25m s^-1), K2-139 is an ideal target to measure the spin-orbit angle of a planetary system hosting a warm Jupiter.