Context : The prospects for using asteroseismology of rapidly oscillating Ap ( roAp ) stars are hampered by the large uncertainty in fundamental stellar parameters .
Results in the literature for the effective temperature ( T _ { eff } ) often span a range of 1000 K .

Aims : Our goal is to reduce systematic errors and improve the T _ { eff } calibration of Ap stars based on new interferometric measurements .

Methods : We obtained long-baseline interferometric observations of \beta CrB using the CHARA/FLUOR instrument .
In order to disentangle the flux contributions of the two components of this binary star , we additionally obtained VLT/NACO adaptive optics images .

Results : We determined limb darkened angular diameters of 0.699 \pm 0.017 mas for \beta CrB A ( from interferometry ) and 0.415 \pm 0.017 mas for \beta CrB B ( from surface brightness-color relations ) , corresponding to radii of 2.63 \pm 0.09 { R } _ { \odot } ( 3.4 % uncertainty ) and 1.56 \pm 0.07 { R } _ { \odot } ( 4.5 % ) .
The combined bolometric flux of the A + B components was determined from satellite UV data , spectrophotometry in the visible and broadband data in the infrared .
The flux from the B component constitutes 16 \pm 4 % of the total flux and was determined by fitting an ATLAS9 model atmosphere to the broad-band NACO J and K magnitudes .
Combining the flux of the A component with its measured angular diameter , we determine the effective temperature T _ { eff } ( { A } ) = 7980 \pm 180 K ( 2.3 % ) .

Conclusions : Our new interferometric and imaging data enable a nearly model-independent determination of the effective temperature of \beta CrB A .
Including our recent study of \alpha Cir , we now have direct T _ { eff } measurements of two of the brightest roAp stars , providing a strong benchmark for an improved calibration of the T _ { eff } scale for Ap stars .
This will support the use of potentially strong constraints imposed by asteroseismic studies of roAp stars .