2015A&A...581A.106G

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. 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. We found that IO Aqr is a hierarchical triple system consisting of a double-lined short-period (P=2.37d) spectroscopic binary and a low-luminosity and low-mass companion star orbiting the binary with a period of >25000d (>70yr) 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±0.004M_☉ and radii R₁=2.19±0.02 and R₂=2.49±0.02R_☉. The companion is most probably a late K-type dwarf with mass ≃0.6M_☉. The distance to the system resulting from applying a (V-K) 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.