  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 to 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 HD 188774 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 HD
188774, 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.
