Context :

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 \gtrsim 25000 d ( \gtrsim 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 M _ { 1 } = 1.569 \pm 0.004 and M _ { 2 } = 1.655 \pm 0.004 M _ { \odot } and radii R _ { 1 } = 2.19 \pm 0.02 and R _ { 2 } = 2.49 \pm 0.02 R _ { \odot } .
The companion is most probably a late K-type dwarf with mass \approx 0.6 M _ { \odot } .
The distance to the system resulting from applying a ( V - K ) surface brightness – colour relation is 255 \pm 6 ( stat . ) \pm 6 ( sys . )
pc , which agrees well with the Hipparcos value of 270 ^ { +91 } _ { -55 } pc , but is more precise by a factor of eight .

Conclusions :