A search for magnetic δ Scuti stars in Kepler hybrid candidates

Numerous candidate hybrid stars of type delta Scuti-gamma Doradus have been identified with the Kepler satellite. However, many of them lie outside the theoretically expected instability strip for hybrid pulsation, where delta Sct and gamma Dor pulsations can be simultaneously excited. We postulate that some of these pulsating stars may not be genuine hybrid pulsators but rather magnetic delta Sct stars, for which the rotational modulation from spots on the surface associated with the magnetic field produces frequencies in the same domain as gamma Dor pulsations. We search for the presence of a magnetic field in a small sample of selected hybrid delta Sct-gamma Dor stars using spectropolarimetry. At the time of observations, the only delta Sct star known to have a magnetic field was HD188774 with a field strength of a few hundred Gauss. Our observations were thus tailored to detect fields of this typical strength. We find no magnetic field in the hybrid candidate stars we observed. However, two of the three other magnetic delta Sct stars discovered since these observations have much weaker fields than HD188774, and are of dynamo origin rather than fossil fields. It is likely that our observations are not sensitive enough to detect such dynamo magnetic fields in the cooler stars of our sample if they are present. This work nevertheless provides reliable upper limits on possible fossil fields in the hotter stars, pointing towards typically weaker fields in delta Sct stars than in OBA stars in general.