  The influence of stellar activity on the fundamental properties of stars
around and below 1 Msun is not well understood. We aim to determine absolute
dimensions and abundances for the solar-type detached eclipsing binary V636
Cen. The results are based on uvby light curves, uvby-beta standard photometry,
radial velocity observations, and high-resolution spectra. Masses and radii
that are precise to 0.5% have been established for the components of V636 Cen.
The 0.85 Msun secondary component is moderately active with starspots and CaII
H and K emission, and the 1.05 Msun primary shows signs of activity as well,
but at a much lower level. We derive a [Fe/H] abundance of -0.20+/-0.08 and
similar abundances for Si, Ca, Ti, V, Cr, Co, and Ni. Corresponding
solar-scaled stellar models are unable to reproduce V636 Cen, especially its
secondary component, which is ~10% larger and ~400 K cooler than predicted.
Models adopting significantly lower mixing-length parameters l/H_p remove these
discrepancies, seen also for other solar-type binary components. For the
observed [Fe/H], Claret models for l/H_p = 1.4 (primary) and 1.0 (secondary)
reproduce the components of V636 Cen at a common age of 1.35 Gyr. V636 Cen and
10 other well-studied inactive and active solar-type binaries suggest that
chromospheric activity, and its effect on envelope convection, is likely to
cause radius and temperature discrepancies, which can be removed by adjusting
the model mixing length parameters downwards. Noting this, the sample may also
lend support to theoretical 2D radiation hydrodynamics studies, which predict a
slight decrease of the mixing length parameter with increasing temperature/mass
for inactive main sequence stars.
