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We present the first detection of the envelope which surrounds the post-AGBbinary source HR 4049. VINCI-VLTI K-band interferometric observations of thissource imply the existence of a large structure with a Gaussian angular FWHM22.4 +/- 1.4 mas or uniform disk diameter of 34.9 +/- 1.9 mas. With theHipparcos parallax of 1.50 +/- 0.64 mas these values correspond to a physicalsize of 14.9 (+11.1,-4.4) AU and 23.3 (+17.3,-7.0) AU, respectively. Ourmeasurements, covering an azimuth range of about 60 degrees, for thesky-projected baseline, provide information on the geometry of the emittingregion and show that there is only a slight variation of the measured angularvalues along the different directions sampled. Thus, our results are consistentwith a spherical geometry of the envelope. However, we cannot completely ruleout the existence of an asymmetric envelope (like the circumbinary diskenvisaged by some recent models) because of the limited spatial frequency andazimuth range covered by the observations.Comment: 4 pages, 4 postscript figures, accepted letter for A&

VINCI-VLTI measurements of HR 4049: The physical size of the circumbinary envelope