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K2-139 b is a warm Jupiter with an orbital period of 28.4 d, but only threetransits of this system have previously been observed, in the long-cadence modeof K2, limiting the precision with which the orbital period can be determined,and future transits predicted. We report photometric observations of fourtransits of K2-139 b with ESA's CHaracterising ExOPlanet Satellite (CHEOPS),conducted with the goal of measuring the orbital obliquity via spot-crossingevents. We jointly fit these CHEOPS data alongside the threepreviously-published transits from the K2 mission, considerably increasing theprecision of the ephemeris of K2-139 b. The transit times for this system cannow be predicted for the next decade with a $1 \sigma$ precision less than 10minutes, compared to over one hour previously, allowing the efficientscheduling of observations with Ariel. We detect no significant deviation froma linear ephemeris, allowing us to exclude the presence of a massive outerplanet orbiting with a period less than 150 d, or a brown dwarf with a periodless than one year. We also determine the scaled semi-major axis, the impactparameter, and the stellar limb-darkening with improved precision. This isdriven by the shorter cadence of the CHEOPS observations compared to that ofK2, and validates the sub-exposure technique used for analysing long-cadencephotometry. Finally, we note that the stellar spot configuration has changedfrom the epoch of the K2 observations; unlike the K2 transits, we detect noevidence of spot-crossing events in the CHEOPS data.

A CHEOPS Search for Massive, Long-Period Companions to the Warm Jupiter K2-139 b