Context : Young close binaries clear central cavities in their surrounding circumbinary disk from which the stars can still accrete material .
This process takes place within the very first astronomical units , and is still not well constrained as the observational evidence has been gathered , until now , only by means of spectroscopy .

Aims : The young object HD 200775 ( MWC 361 ) is a massive spectroscopic binary ( separation of \sim 15.9 mas , \sim 5.0 AU ) , with uncertain classification ( early/late Be ) , that shows a strong and variable H _ { \alpha } emission .
We aim to study the mechanisms that produce the H _ { \alpha } line at the AU-scale .

Methods : Combining the radial velocity measurements and astrometric data available in the literature , we determined new orbital parameters .
With the VEGA instrument on the CHARA array , we spatially and spectrally resolved the H _ { \alpha } emission of HD 200775 on a scale of a few milliarcseconds , at low and medium spectral resolutions ( R \sim 1600 and 5000 ) over a full orbital period ( \sim 3.6 years ) .

Results : We observe that the H _ { \alpha } equivalent width varies with the orbital phase , and increases close to periastron , as expected from theoretical models that predict an increase of the mass transfer from the circumbinary disk to the primary disk .
In addition , using spectral visibilities and differential phases , we find marginal variations of the typical extent of the H _ { \alpha } emission ( at 1 to 2 \sigma level ) and location ( at 1 to 5 \sigma level ) .
The spatial extent of the H _ { \alpha } emission , as probed by a Gaussian FWHM , is minimum at the ascending node ( 0.67 \pm 0.20 mas , i.e. , 0.22 \pm 0.06 AU ) , and more than doubles at periastron .
In addition , the Gaussian photocenter is slightly displaced in the direction opposite to the secondary , ruling out the scenario in which all or most of the H _ { \alpha } emission is due to accretion onto the secondary .
These findings , together with the wide H _ { \alpha } line profile , may be due to a non-spherical wind enhanced at periastron .

Conclusions : For the first time in a system of this kind , we spatially resolve the H _ { \alpha } line and estimate that it is emitted in a region larger than the one usually inferred in accretion processes .
The H _ { \alpha } line could be emitted in a stellar or disk-wind , enhanced at periastron as a result of gravitational perturbation , after a period of increased mass accretion rate .
Our results suggest a strong connection between accretion and ejection in these massive objects , consistent with the predictions for lower-mass close binaries .