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 (Teq = 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|MJ and radius of |0.808 - 0.033 ^ + 0.034|RJ, 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.