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+0.083−0.075MJ and radius of 0.808+0.034−0.033RJ, K2-139 b is one of the transiting warm Jupiters with the lowest mass known to date. The planetary mean density of 0.91+0.24−0.20 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.