On the Contradictory Case of the Binary System HD 81809 Hosting Two Pulsating Solar-like Stars Observed by TESS

We present a new comprehensive study of HD 81809, a nearby binary system of two solar-like stars showing high-amplitude X-ray emission and a well-defined 8 yr solar-like magnetic cycle. By analyzing high-resolution spectroscopy, alongside Gaia Data Release 3 (DR3) astrometry and bolometric fluxes, we derive updated fundamental parameters for both components. In particular, we uncover a significant chemical difference: the primary is metal poor ([Fe/H] similar or equal to -0.57), while the secondary shows solar-like metallicity ([Fe/H] similar or equal to 0.00). This suggests that the system originated in a mildly metal-poor environment, consistent with the Galactic thick disk population, and that the secondary's surface composition has been altered by a recent accretion event. Using multisector TESS photometry, we detected solar-like oscillations in both components, deriving global asteroseismic parameters Delta nu = 43.32 +/- 3.91 mu Hz and nu(max)=708.74(-3.74)(+3.23)mu Hz for HD 81809 A, and Delta nu = 97.75 +/- 4.49 mu Hz and nu(max)=2098.07-2.83+3.07 mu Hz for HD 81809 B. By combining all the observational constraints with stellar evolutionary models computed using the CLES and MESA codes, we reconstructed the evolutionary scenario of the system. Our results indicate that HD 81809 is an old system with an age of similar to 10 Gyr, composed of a subgiant primary with mass M similar to 0.87 (circle dot) and radius R similar to 1.96 R-circle dot-likely responsible for the reactivated dynamo cycle-and a main-sequence secondary with mass M similar to 0.85 M-circle dot and radius R similar to R 1.10 R-circle dot. This system represents a benchmark for studying stellar evolution, magnetic activity, and the physics of old, metal-poor stars in the Galactic thick disk.