Context : The main sequence binary star 61 Cyg ( K5V+K7V ) is our nearest stellar neighbour in the northern hemisphere .
This proximity makes it a particularly well suited system for very high accuracy interferometric radius measurements .

Aims : Our goal is to constrain the poorly known evolutionary status and age of this bright binary star .

Methods : We obtained high accuracy interferometric observations in the infrared K ^ { \prime } band , using the CHARA/FLUOR instrument .
We then computed evolutionary models of 61 Cyg A & B with the CESAM2k code .
As model constraints , we used a combination of observational parameters from classical observation methods ( photometry , spectroscopy ) as well as our new interferometric radii .

Results : The measured limb darkened disk angular diameters are \theta _ { LD } ( A ) = 1.775 \pm 0.013 mas and \theta _ { LD } ( B ) = 1.581 \pm 0.022 mas , respectively for 61 Cyg A and B .
Considering the high accuracy parallaxes available , these values translate into photospheric radii of { R } ( A ) = 0.665 \pm 0.005 { R } _ { \odot } and { R } ( B ) = 0.595 \pm 0.008 { R } _ { \odot } .
The new radii constrain efficiently the physical parameters adopted for the modeling of both stars , allowing us to predict asteroseismic frequencies based on our best-fit models .

Conclusions : The CESAM2k evolutionary models indicate an age around 6 Gyrs and are compatible with small values of the mixing length parameter .
The measurement of asteroseismic oscillation frequencies in 61 Cyg A & B would be of great value to improve the modeling of this important fiducial stellar system , in particular to better constrain the masses .