Aims: We study the Be star δ Cen circumstellar disk using long-baseline interferometry which is the only observing technique capable of resolving spatially and spectroscopically objects smaller than 5 mas in the H and K band. <BR />Methods: We used the VLTI/AMBER instrument on January 6, 8, and 9, 2008, in the H and K bands to complete low (35) and medium (1500) spectral resolution observations. <BR />Results: We detected an oscillation in the visibility curve plotted as a function of the spatial frequency which is a clear signature of a companion around δ Cen. Our best-fit soltution infers a binary separation of 68.7 mas, a companion flux contribution in the K band of about 7% of the total flux, a PA of 117.5°, and an envelope flux around the Be primary that contributes up to about 50% of the total flux, in agreement with our Spectral Energy Distribution (SED) fit. The envelope size is estimated to be 1.6 mas in K but no departure from spherical symmetry is detected. <P />Based on observations made with ESO telecopes at La Silla Paranal Observatory under GTO programme IDs 080.D-0131(A) and 080.C-0099(B).
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