The nature of EUPegasi: An Algol-type binary with a δ Scuti-type component.

The comprehensive photometry and spectroscopy for the neglected eclipsing binary EUPegasi are presented. We determine its spectral type to be A3V. With the W-D program, the photometric solution was deduced from the four-color light curves. The results imply that EUPeg is a detached binary with a mass ratio of q = 0.3105(± 0.0011), whose components nearly fill their Roche lobes. The low-amplitude pulsation occurs around the secondary eclipse, which may be attributed to the more massive component. Three frequencies are preliminarily explored by the Fourier analysis. The pulsating frequency at f1 = 34.1 c d^-1 is a p-mode pulsation. The orbital period may be undergoing a secular decrease, superimposed by a cyclic variation. The period decreases at a rate of dP/dt = -7.34 ± 1.06 d yr^-1, which may be attributed to mass loss from the system due to stellar wind. The cyclic oscillation, with Pmod = 31.0 ± 1.4 yr and A = 0.0054 ± 0.0010 d, may be caused by the light-time effect due to the assumed third body. With its evolution, the pulsating binary EUPeg will evolve from the detached configuration to the semi-detached case. [ABSTRACT FROM AUTHOR]