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Context. While exoplanets are now routinely detected, the detection of small bodies in extrasolar systems remains challenging. Since the discovery of sporadic events, which are interpreted to be exocomets (falling evaporating bodies) around β Pic in the early 1980s, only ∼20 stars have been reported to host exocomet-like events. Aims. We aim to expand the sample of known exocomet-host stars, as well as to monitor the hot-gas environment around stars with previously known exocometary activity. Methods. We have obtained high-resolution optical spectra of a heterogeneous sample of 117 main-sequence stars in the spectral type range from B8 to G8. The data were collected in 14 observing campaigns over the course of two years from both hemispheres. We analysed the Ca » II K&H and Na » I D lines in order to search for non-photospheric absorptions that originated in the circumstellar environment and for variable events that could be caused by the outgassing of exocomet-like bodies. Results. We detected non-photospheric absorptions towards 50% of the sample, thus attributing a circumstellar origin to half of the detections (i.e. 26% of the sample). Hot circumstellar gas was detected in the metallic lines inspected via narrow stable absorptions and/or variable blue- and red-shifted absorption events. Such variable events were found in 18 stars in the Ca » II and/or Na » I lines; six of them are reported in the context of this work for the first time. In some cases, the variations we report in the Ca » II K line are similar to those observed in β Pic. While we do not find a significant trend in the age or location of the stars, we do find that the probability of finding CS gas in stars with larger v sin i is higher. We also find a weak trend with the presence of near-infrared excess and with anomalous (λ Boo-like) abundances, but this would require confirmation by expanding the sample. ; With funding from the Spanish government through the "María de Maeztu Unit of Excellence" accreditation (MDM-2017-0737)

This is a pre-copyedited, author-produced PDF of an article accepted for publication in Monthly Notices of the Royal Astronomical Society following peer review. The version of record Barragán, O. et al. "K2-139 b: A low-mass warm Jupiter on a 29-d orbit transiting an active K0 V star" is available online at: http://doi.org/10.1093/mnras/stx3207 ; 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 (logR'HK=-4.46±0.06) K0V star in K2 Campaign 7.We derive the system's parameters by combining the K2 photometry with groundbased follow-up observations. With a mass of 0.387-0.033+0.075MJand radius of 0.808-0.033+0.034RJ, 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+0.24g cm-3can be explained with a core of ~50M⊕. 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 ; D. Gandolfi gratefully acknowledges the financial support of the Programma Giovani Ricercatori – Rita Levi Montalcini – Rientro dei Cervelli (2012) awarded by the Italian Ministry of Education, Universities and Research (MIUR). Sz. Csizmadia thanks the Hungarian OTKA Grant K113117. H. J. Deeg and D. Nespral acknowledge support by grant ESP2015-65712-C5-4-R of the Spanish Secretary of State for R& D&i (MINECO). D. Lorenzo-Oliveira acknowledges the support from FAPESP (2016/20667-8). This research was supported by the Ministerio de Economia y Competitividad under project FIS2012-31079. The research leading to these results has received funding from the European Union Seventh Framework Programme (FP7/2013-2016) under grant agreement No. 312430 (OPTICON).