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Strong magnetic fields in chemically peculiar A-type (Ap) stars typicallysuppress low-overtone pressure modes (p modes) but allow high-overtone p modesto be driven. KIC 11296437 is the first star to show both. We obtained andanalysed a Subaru spectrum, from which we show that KIC 11296437 has abundancessimilar to other magnetic Ap stars, and we estimate a mean magnetic fieldmodulus of $2.8\pm0.5$ kG. The same spectrum rules out a double-linedspectroscopic binary, and we use other techniques to rule out binarity over awide parameter space, so the two pulsation types originate in one $\delta$Sct--roAp hybrid pulsator. We construct stellar models depleted in helium anddemonstrate that helium settling is second to magnetic damping in suppressinglow-overtone p modes in Ap stars. We compute the magnetic damping effect forselected p and g modes, and find that modes with frequencies similar to thefundamental mode are driven for polar field strengths $\lesssim4$ kG, whileother low-overtone p modes are driven for polar field strengths up to $\sim$1.5kG. We find that the high-order g modes commonly observed in $\gamma$ Dor starsare heavily damped by polar fields stronger than 1--4 kG, with the dampingbeing stronger for higher radial orders. We therefore explain the observationthat no magnetic Ap stars have been observed as $\gamma$ Dor stars. We use ourhelium-depleted models to calculate the $\delta$ Sct instability strip formetallic-lined A (Am) stars, and find that driving from a Rosseland meanopacity bump at $\sim$$5\times10^4$ K caused by the discontinuous H-ionizationedge in bound-free opacity explains the observation of $\delta$ Sct pulsationsin Am stars.

On the first $\delta$ Sct--roAp hybrid pulsator and the stability of p and g modes in chemically peculiar A/F stars