THE BINARY SYSTEM Σ2173

New high-dispersion spectroscopic observations of the visual binary Σ2173 are combined with earlier ones obtained during passage through the opposite node, and with the available visual and interferometric observations, to determine the orbital elements completely. We find P =46y.43, T=1962.52, Ω = 152°.7, ω=98°.7, ω=147°.6, α" = 0°.97, V₀ = -76.98 km s⁻¹ and K₂ = 4.84 km s⁻¹. The parallax derived is O". O59, somewhat larger than the best trigonometrical value of O". O5, which leads to masses and luminosities for the components in good agreement with the spectral types and luminosity classes but which are problematical from the point of view of theoretical studies of stellar evolution. The two stars are very similar: We can distinguish no difference in spectral type and find Δm to be approximately 0ṃ1. The brighter star appears to be the less massive.