  We present the first detection of the envelope which surrounds the post-AGB
binary source HR 4049. VINCI-VLTI K-band interferometric observations of this
source imply the existence of a large structure with a Gaussian angular FWHM
22.4 +/- 1.4 mas or uniform disk diameter of 34.9 +/- 1.9 mas. With the
Hipparcos parallax of 1.50 +/- 0.64 mas these values correspond to a physical
size of 14.9 (+11.1,-4.4) AU and 23.3 (+17.3,-7.0) AU, respectively. Our
measurements, covering an azimuth range of about 60 degrees, for the
sky-projected baseline, provide information on the geometry of the emitting
region and show that there is only a slight variation of the measured angular
values along the different directions sampled. Thus, our results are consistent
with a spherical geometry of the envelope. However, we cannot completely rule
out the existence of an asymmetric envelope (like the circumbinary disk
envisaged by some recent models) because of the limited spatial frequency and
azimuth range covered by the observations.
