KIC 8840638: A new eclipsing binary consisting of $δ$ Scuti-type oscillations with an extremely cold companion star

In this paper, we analyze the light variation of KIC 8840638 using high-precision time-series data from $Kepler$ mission. The analysis reveals this target is a new Algol-type eclipsing binary system with a $δ$ Scuti component, not a pure single $δ$ Scuti star previously known. The frequency analysis of the short-cadence light curve reveals 95 significant frequencies, most of which lie in a frequency range of 23$-$32 d$^{-1}$. Among them, seven independent frequencies are detected in the typical frequency range of $δ$ Scuti stars and they are identified as pressure modes. Besides, the orbital frequency $f_{orb}$ (=0.320008 d$^{-1}$) and its harmonics are also detected directly in the frequency spectrum. The binary modellings 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 $q$ = 0.454, an inclination angle of 58 degrees, and a temperature difference of larger than 4000 K between the two components. The derived parameters suggest the primary of this system is a main-sequence star with spectral type about A7V, while the secondary seems to be the coolest companion star among the known semi-detached Algol systems, implying it may have stepped into a highly evolved stage.

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