Observable log (energy) plotted versus log (efficiency) for δ Sct (open squares) and γ Dor (asterisks) stars only (top), and for hybrid stars as well (bullets) (bottom). The left panels include all 480 Kepler stars that are assigned to one of the three groups. The right panels show the 49 stars for which reliable values of T_eff and log g are available. In the on-line version of the paper the symbols representing the δ Sct, γ Dor and hybrid stars are red, blue, and black, respectively. The cross in the right bottom corner of the top left panel represents the typical error bars on the values: 0.04 dex and 0.12 dex for log (energy) and log (efficiency), respectively.