Accurate Orbital Solution for the New and Metal-poor Eclipsing Binary Tycho 5227-1023-1

The orbit and physical parameters of the previously unsolved double-lined eclipsing binary Tyc 5227-1023-1, discovered during the search for RR Lyr variable candidate members of the Aquarius stream, are derived using high-resolution chelle spectroscopy and V, i' photometry. A synthetic spectral analysis of both components has been performed, yielding metallicity [M/H] =-0.63± 0.11 for both stars and a temperature for the secondary that is in close agreement with the one from the orbital solution, while the temperature of the primary is determined from photometry (T₁ = 6350 K). The masses and radii (M₁= 0.96 ± 0.02, M₂ = 0.84 ± 0.01 M_⊙, R₁ = 1.39 ± 0.01, R₂ = 0.98 ± 0.01 R_⊙) reveal that both stars have already slightly evolved away from the main-sequence band, having an age of about 7 Gyr, and the results of the synthetic spectral analysis support the claim of corotation with the orbital motion. The radial velocity of the system is -60 ± 2 km s^-1, while its distance, computed from orbital parameters and the derived reddening E_B-V = 0.053, is 496± 35 pc. Even though Tyc 5227-1023-1 was initially treated as a possible member of the Aquarius stream, the results presented here disagree with reported values for this ancient structure and suggest a likely membership of the thick disk.