The Araucaria project. Precise physical parameters of the eclipsing binary IO Aquarii.

Aims. Our aim is to precisely measure the physical parameters of the eclipsing binary IO Aqr and derive a distance to this system by applying a surface brightness -- colour relation. Our motivation is to combine these parameters with future precise distance determinations from the Gaia space mission to derive precise surface brightness -- colour relations for stars. Methods. We extensively used photometry from the Super-WASP and ASAS projects and precise radial velocities obtained from HARPS and CORALIE high-resolution spectra. We analysed light curves with the code JKTEBOP and radial velocity curves with the Wilson-Devinney program. Results. We found that IO Aqr is a hierarchical triple system consisting of a double-lined short-period (P = 2.37 d) spectroscopic binary and a low-luminosity and low-mass companion star orbiting the binary with a period of ≳25 000 d (≳70 yr) on a very eccentric orbit. We derive high-precision (better than 1%) physical parameters of the inner binary, which is composed of two slightly evolved main-sequence stars (F5 V-IV + F6 V-IV) with masses of M1 = 1.569 ± 0.004 and M2 = 1.655 ± 0.004 M⊙ and radii R1 = 2.19 ± 0.02 and R2 = 2.49 ± 0.02 R⊙. The companion is most probably a late K-type dwarf with mass ≈0.6 M⊙. The distance to the system resulting from applying a () surface brightness -- colour relation is 255 ± 6 (stat.) ± 6 (sys.) pc, which agrees well with the HIPPARCOS value of 270+91-55 pc, but is more precise by a factor of eight. [ABSTRACT FROM AUTHOR]