We present observations of the famous OH/IR star OH26.5+0.6 obtained using the Mid-Infrared Interferometric Instrument MIDI at the European Southern Observatory ( ESO ) Very Large Telescope Interferometer VLTI .
The emission of the dusty envelope , spectrally dispersed at a resolution of 30 from 8Â \mu m to 13.5Â \mu m , appears resolved by a single dish UT telescope .
In particular the angular diameter increases strongly within the silicate absorption band .
Moreover an acquisition image taken at 8.7Â \mu m exhibits , after deconvolution , a strong asymmetry .
The axis ratio is 0.75 \pm 0.07 with the FWHM of the major and minor axis which are 286Â mas and 214Â mas respectively .
The measured PA angle , 95 ^ { \circ } \pm 6 ^ { \circ } is reminiscent of the asymmetry in the OH maser emission detected at 1612MHz by Bowers & Johnston ( 1990 ) for this star .
In interferometric mode the UT1-UT3 102m baseline was employed to detect the presence of the star .
No fringes have been found with a detection threshold estimated to be of the order of 1 % of the total flux of the source , i.e .
5-8Â Jy .
These observations were carried out during the phase of maximum luminosity of the star , when the dust shell is more diluted and therefore the chance to detect the central source maximized .
We modeled the dusty environment based on the work of Justannont et al .
( 1996 ) .
In particular , the failure to detect fringes provides strong constraints on the opacities in the inner regions of the dust shell or in the close vicinity of the star .