Time-series spectroscopy of the pulsating eclipsing binary XX Cephei
Jae-Rim Koo (Korea Astronomy and Space Science Institute), Jae Woo Lee (Korea Astronomy and Space Science Institute), Kyeongsoo Hong (Korea Astronomy and Space Science Institute), Seung-Lee Kim (Korea Astronomy and Space Science Institute), et al.

Oscillating Algol-type eclipsing binaries (oEA) are very interesting objects that have three observational features of eclipse, pulsation, and mass transfer. Direct measurement of their masses and radii from the double-lined radial velocity data and photometric light curves would be the most essential for understanding their evolutionary process and for performing the asteroseismological study. We present the physical properties of the oEA star XX Cep from high-resolution time-series spectroscopic data. The effective temperature of the primary star was etermined to be 7,946 +/- 240 K by comparing the observed spectra and the Kurucz models. We detected the absorption lines of the secondary star, which had never been detected in previous studies, and obtained the radial velocities for both components. With the published BVRI light curves, we determined the absolute parameters for the binary via Wilson-Devinney modeling. The masses and radii are M1 = 2.49 +/- 0.06 Msun, M2 = 0.38 +/- 0.01 Msun, R1 = 2.27 +/- 0.02 Rsun, and R2 = 2.43 +/- 0.02 Rsun, respectively. The primary star is about 45 % more massive and 60 % larger than the zero-age main sequence (ZAMS) stars with the same effective temperature. It is probably because XX Cep has experienced a very different evolutionary process due to mass transfer, contrasting with the normal main sequence stars. The primary star is located inside the theoretical instability strip of delta Sct-type stars on HR diagram. We demonstrated that XX Cep is an oEA star, consisting of a delta Sct-type pulsating primary component and an evolved secondary companion.