| Résumé | We separate and analyse the component spectra of the composite-spectrum binary HD 208253. We find that the cool primary is an evolving star of spectral type G7 III, while its hot secondary is an early-A dwarf. The giant is currently near the lowest point of the red-giant branch and is slightly less luminous than its dwarf companion. We provide a set of precise radial-velocity measurements for both stars. The double-lined orbit which we derive from them shows that the component mass ratio is close to unity (q = 1.05 ± 0.01). We deduce the physical properties of both stars, determine their respective masses to be 2.75 ± 0.07 Me (giant) and 2.62 ± 0.07 Me (dwarf), and show that the orbit's inclination is within a degree or two of 68°. The spectrum of the A-type component has quite component has quite narrow lines (we infer a rotational velocity of 18 km s-1), though since the period of the orbit is well over 1 year that component cannot be in synchronous rotation. An intriguing property of the dwarf is its enhanced Sr and Ba, though it does not exhibit the other spectral peculiarities that would signal a classical Am star. While by no means unique amongst the multitude of oddities exhibited by A and early-F stars, this dwarf which we have uncovered in a long-period binary offers valuable constraints and challenges to stellar-evolution theory. |
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