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We report magnetic field measurements spanning about 15 yr of four massive (7.5–15 M⊙) supergiant stars: α Per (HD 20902, F5Iab), α Lep (HD 36673A, F0Ib), η Leo (HD 87737, A0Ib) and 13 Mon (HD 46300, A1Ib). For each star, spectropolarimetric observations were collected using ESPaDOnS at the Canada–France–Hawaii Telescope. The observed spectra were coadded, normalized, and then processed using least-squares deconvolution to yield mean Stokes I and V profiles. Each spectrum was analyzed to infer the false-alarm probability of signal detection, and the longitudinal magnetic field was measured. This process yielded persistent detection of magnetic fields in all four stars. The median 1σ longitudinal field uncertainty of the Zeeman detections was 0.6 G. The maximum unsigned longitudinal magnetic fields measured from the detections are rather weak, ranging from 0.34  ±  0.19 G for α Lep to 2.61  ±  0.55 G for 13 Mon. The Zeeman signatures show different levels of complexity; those of the two hotter stars are relatively simple, while those of the two cooler stars are more complex. The stars also exhibited different levels of variability of their Zeeman signatures and longitudinal fields. We report periodic variability of the longitudinal field and (complex) Stokes V profiles of α Per with a period of either 50.75 days or 90 days. The (simple) Stokes V profiles of 13 Mon, and probably those of η Leo, show global polarity changes once during the period of observation, but the data are insufficient to place strong constraints on the variability timescales. 

Abstract PLATO (PLAnetary Transits and Oscillations of stars) is ESA’s M3 mission designed to detect and characterise extrasolar planets and perform asteroseismic monitoring of a large number of stars. PLATO will detect small planets (down to <2R$$_\textrm{Earth}$$ $_{\text{Earth}}^{}$) around bright stars (<11 mag), including terrestrial planets in the habitable zone of solar-like stars. With the complement of radial velocity observations from the ground, planets will be characterised for their radius, mass, and age with high accuracy (5%, 10%, 10% for an Earth-Sun combination respectively). PLATO will provide us with a large-scale catalogue of well-characterised small planets up to intermediate orbital periods, relevant for a meaningful comparison to planet formation theories and to better understand planet evolution. It will make possible comparative exoplanetology to place our Solar System planets in a broader context. In parallel, PLATO will study (host) stars using asteroseismology, allowing us to determine the stellar properties with high accuracy, substantially enhancing our knowledge of stellar structure and evolution. The payload instrument consists of 26 cameras with 12cm aperture each. For at least four years, the mission will perform high-precision photometric measurements. Here we review the science objectives, present PLATO‘s target samples and fields, provide an overview of expected core science performance as well as a description of the instrument and the mission profile towards the end of the serial production of the flight cameras. PLATO is scheduled for a launch date end 2026. This overview therefore provides a summary of the mission to the community in preparation of the upcoming operational phases.