First discovery of a magnetic field in a main-sequence δ Scuti star: the Kepler star HD 188774

The Kepler space mission provided a wealth of delta Sct-gamma Dor hybrid candidates. While some may be genuine hybrids, others might be misclassified due to the presence of a binary companion or to rotational modulation caused by magnetism and related surface inhomogeneities. In particular, the Kepler delta Sct-gamma Dor hybrid candidate HD 188774 shows a few low frequencies in its light and radial velocity curves, whose origin is unclear. In this work, we check for the presence of a magnetic field in HD 188774. We obtained two spectropolarimetric measurements with an Echelle SpectroPolarimetric Device for the Observation of Stars (ESPaDOnS) at Canada-France-Hawaii Telescope. The data were analysed with the leastsquares deconvolution (LSD) method. We detected a clear magnetic signature in the Stokes V LSD profiles. The origin of the low frequencies detected in HD 188774 is therefore most probably the rotational modulation of surface spots possibly related to the presence of a magnetic field. Consequently, HD 188774 is not a genuine hybrid delta Sct-gamma Dor star, but the first known magnetic main-sequence delta Sct star. This makes it a prime target for future asteroseismic and spot modelling. This result casts new light on the interpretation of the Kepler results for other delta Sct-gamma Dor hybrid candidates.