* Astronomy

Title: EPIC 218916923 b: a low-mass warm Jupiter on a 29-day orbit transiting an active K0 V star
Author: O. Barragán, D. Gandolfi, A. M. S. Smith, H. J. Deeg, M. C. V. Fridlund, C. M. Persson, P. Donati, M. Endl, Sz. Csizmadia, S. Grziwa, D. Nespral, A. P. Hatzes, W. D. Cochran, L. Fossati, S. S. Brems, J. Cabrera, F. Cusano, Ph. Eigmüller, C. Eiroa, A. Erikson, E. Guenther, J. Korth, L. Mancini, M. Pätzold, J. Prieto-Arranz, H. Rauer, I. Rebollido, J. Saario, O.V. Zakhozhay

We announce the discovery of EPIC218916923 b, a transiting warm-Jupiter (T_eq=555±11 K) on a 29-day orbit around an active (logR'_HK = -4.49 ± 0.03) 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 M_p=0.381±0.045M_Jup and radius of R_p=0.812±0.030RJup, EPIC218916923 b is one of the transiting warm Jupiters with the lowest mass known to date. The planetary mean density rho_p=0.88±0.14gcm^-3 can be explained with a core of ~50 Earth masses. Given the brightness of the host star (V= 11.653 mag), the relatively short transit duration (~5 hours), and the expected amplitude of the Rossiter-McLaughlin effect (~25 ms), EPIC218916923 is an ideal target to measure the spin-orbit angle of a planetary system hosting a warm Jupiter.

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