Context :

Aims : We investigate the molecular and dusty environment of OH/IR stars in order to characterize the mass-loss process during the tip-AGB superwind phase .

Methods : Employing the AMBER instrument at the VLT Interferometer we obtained near-infrared H - and K -band spectro-interferometric observations of the three OH/IR stars IRAS~13479-5436 , IRAS~14086-6907 and IRAS~17020-5254 with a spectral resolution of about 35 .
We use a two-component geometrical model , consisting of a uniform disk and a Gaussian disk , to obtain characteristic angular sizes of the central stellar sources and their dust envelopes , as well as the flux ratios between these components .

Results : Angular uniform disk diameters of the three central components of the objects above have values between 3.2 mas and 5.4 mas .
For their dust envelopes , we find FWHM values between 17.1 mas and 25.2 mas .
The three objects show significantly different flux contributions of the shells to the total near-IR flux of 61 % , 38 % , and 16 % for IRAS 13479-5436 , IRAS 14086-6907 , and IRAS 17020-5254 , respectively .
According to distance estimates from the literature , the central stellar components have radii between 900 R _ { \odot } and 1400 R _ { \odot } , while their dust envelopes reach FWHM values between 9000 R _ { \odot } and 13000 R _ { \odot } .
The visibility functions of all three sources exhibit wavelength variations that resemble those of earlier VLTI/AMBER observations of semi-regular and Mira variable AGB stars .
These are interpreted as characteristic of atmospheric molecular layers lying above the photosphere .

Conclusions : The derived characteristic sizes of both , the central stellar atmospheres and dust envelopes are consistent with the canonical properties of OH/IR stars .
The spectral visibility variations resemble those of other AGB stars and indicate the presence of molecular layers , confirming that these are a common phenomenon among AGB stars of very different luminosities and mass-loss rates , alike .
We also find that the dust envelopes have a clearly larger optical depth than those known for Mira stars .
We interpret this as an expected result of the “ superwind ” phase , the final 10 000 to 30 000 years of AGB-evolution , when the mass-loss rate increases by a factor of 10-100 .
By their different optical depths , the three dust shells studied here may represent different stages of the “ superwind ” and different initial masses .