KIC 8840638: A high mass-ratio eclipsing binary consisting of a $\delta$ Scuti star with multiperiodic pulsations.

In this paper, we analyse the light variation of KIC 8840638 using high-precision time-series data delivered by the $Kepler$ mission. The analysis reveals that the target KIC 8840638 is an eclipsing binary with a $\delta$ Scuti component, not a pure single $\delta$ Scuti star previously known. The frequency analysis of the high-precision short cadence light curve reveals 95 significant frequencies, most of which lie in the frequency range of 23$-$32 d$^{-1}$. Among them, seven independent frequencies are detected in the typical frequency range of the $\delta$ Scuti variables. Besides, the orbital frequency $f_{orb}$ (=0.320008 d$^{-1}$) and its harmonics are also detected directly in the frequency spectrum. The binary modelings derived from the Wilson-Devinney code indicate the binary parameters are not affected by the pulsations, and the binary system is in semi-detached configure with a mass ratio of 1.6, which is higher than that of all other $\delta$ Scuti eclipsing binaries. The locations of the components of KIC 8840638 indicate the primary star lies in the $\delta$ Scuti instability region below the terminal-age main-sequence, and the secondary component might be an evolved star.