Discovery and Seismic Analysis of an EL CVn-type Binary with Hybrid δ Sct-γ Dor Pulsations

We report the discovery of an EL CVn-type binary consisting of a low-mass, pre-helium white dwarf and the first detection of hybrid δ Sct-γ Dor pulsations in such binary systems. Based on the four years Kepler data, we determined comprehensive photometric solution of the eclipsing binary KIC 8113154. The light-curve modeling reveals that it is a detached system containing a thermally bloated, low-mass, pre-He white dwarf with the mass of 0.26 ± 0.02 M _⊙ and the radius of 0.13 ± 0.01 R _⊙. After removal of the binary model from the observed Kepler data, multiple frequency analysis is applied to the light residuals. The Fourier spectrum shows low-order p-modes and high-order g-mode pulsations that very likely stem from the F-type primary component star, which could be classified as a new δ Sct-γ Dor hybrid. We detected 111 frequencies with signal-to-noise amplitude ratios larger than 4.0. From these frequencies, we identified 17 high-order quadrupole (l = 2) g modes, including eight zonal (m = 0) and nine prograde (m = 2) ones, on the basis of which we derived the asymptotic period spacing of the g modes and the internal rotation rate of the convective core. This is significantly smaller than the orbital frequency, indicating that the core and envelope of the primary star in KIC 8113154 rotate differentially.