The Dynamical Mass and Evolutionary Status of the Type II Cepheid in the Eclipsing Binary System OGLE-LMC-T2CEP-211 with a Double-ring Disk

We present the analysis of a peculiar W Virginis (pWVir) type II Cepheid, OGLE-LMC-T2CEP-211 (P <SUB>puls</SUB> = 9.393 days), in a double-lined binary system (P <SUB>orb</SUB> = 242 days), which shed light on virtually unknown evolutionary status and structure of pWVir stars. The dynamical mass of the Cepheid (first ever for a type II Cepheid) is 0.64 ± 0.02 M <SUB>⊙</SUB>, and the radius R = 25.1 ± 0.3 R <SUB>⊙</SUB>. The companion is a massive (5.67 M <SUB>⊙</SUB>) main-sequence star obscured by a disk. Such a configuration suggests a mass transfer in the system history. We found that originally the system ({P}<SUB>orb</SUB>}<SUP>init</SUP>} = 12 days) was composed of 3.5 and 2.8 M <SUB>⊙</SUB> stars, with the current Cepheid being more massive. The system age is now ˜200 Myr, and the Cepheid is almost completely stripped of hydrogen, with helium mass of ˜92% of the total mass. It finished transferring the mass 2.5 Myr ago and is evolving toward lower temperatures passing through the instability strip. Comparison with observations indicates a reasonable 2.7 × 10<SUP>-8</SUP> M <SUB>⊙</SUB> yr<SUP>-1</SUP> mass loss from the Cepheid. The companion is most probably a Be main-sequence star with T = 22,000 K and R = 2.5 R <SUB>⊙</SUB>. Our results yield a good agreement with a pulsation theory model for a hydrogen-deficient pulsator, confirming the described evolutionary scenario. We detected a two-ring disk (R <SUB>disk</SUB> ˜ 116 R <SUB>⊙</SUB>) and a shell (R <SUB>shell</SUB> ˜ 9 R <SUB>⊙</SUB>) around the companion, which is probably a combination of the matter from the past mass transfer, the mass being lost by the Cepheid owing to wind and pulsations, and a decretion disk around a rapidly rotating secondary. Our study, together with observational properties of pWVir stars, suggests that the majority of them are products of a similar binary evolution interaction.