Mass and p-factor of the Type II Cepheid OGLE-LMC-T2CEP-098 in a Binary System∗ ∗ This paper includes data gathered with the 6.5 m Magellan Clay Telescope at Las Campanas Observatory, Chile

We present the results of a study of the type II Cepheid (Ppuls = 4.974 days) in the eclipsing binary system OGLE-LMC-T2CEP-098 (Porb = 397.2 days). The Cepheid belongs to the peculiar W Vir group, for which the evolutionary status is virtually unknown. It is the first single-lined system with a pulsating component analyzed using the method developed by Pilecki et al. We show that the presence of a pulsator makes it possible to derive accurate physical parameters of the stars even if radial velocities can be measured for only one of the components. We have used four different methods to limit and estimate the physical parameters, eventually obtaining precise results by combining pulsation theory with the spectroscopic and photometric solutions. The Cepheid radius, mass, and temperature are , , and , respectively, while its companion has a similar size ( ), but is more massive ( ) and hotter (9500 K). Our best estimate for the p-factor of the Cepheid is . The mass, position on the period-luminosity diagram, and pulsation amplitude indicate that the pulsating component is very similar to the Anomalous Cepheids, although it has a much longer period and is redder in color. The very unusual combination of the components suggest that the system has passed through a mass-transfer phase in its evolution. More complicated internal structure would then explain its peculiarity.