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Observations of the pulsations of stars can be used to infer their interiorstructure and test theoretical models. The main sequence $\gamma$ Doradus (Dor)and $\delta$ Scuti (Sct) stars with masses 1.2-2.5 $M_{\sun}$ are particularlyuseful for these studies. The $\gamma$ Dor stars pulsate in high-order $g$modes with periods of order 1 day, driven by convective blocking at the base oftheir envelope convection zone. The $\delta$ Sct stars pulsate in low-order $g$and $p$ modes with periods of order 2 hours, driven by the $\kappa$ mechanismoperating in the Heii ionization zone. Theory predicts an overlap region in theHertzsprung-Russell diagram between instability regions, where 'hybrid' starspulsating in both types of modes should exist. The two types of modes withproperties governed by different portions of the stellar interior providecomplementary model constraints. Among the known $\gamma$ Dor and $\delta$ Sctstars, only four have been confirmed as hybrids. Now, analysis of combinedQuarter 0 and Quarter 1 Kepler data for hundreds of variable stars shows thatthe frequency spectra are so rich that there are practically no pure $\delta$Sct or $\gamma$ Dor pulsators, i.e. essentially all of the stars showfrequencies in both the $\delta$ Sct and $\gamma$ Dor frequency range. A newobservational classification scheme is proposed that takes into account theamplitude as well as the frequency, and is applied to categorize 234 stars as$\delta$ Sct, $\gamma$ Dor, $\delta$ Sct/$\gamma$ Dor or $\gamma$ Dor/$\delta$Sct hybrids.

Hybrid gamma Doradus - delta Scuti pulsators: New insights into the physics of the oscillations from Kepler observations