Aims. We study the optical spectral properties of a sample of stars showing far infrared colours similar to those of well-known planetary nebulae. The large majority of them were unidentified sources or poorly known in the literature at the time when this spectroscopic survey started, some 15 years ago. Methods. We present low-resolution optical spectroscopy, finding charts and improved astrometric coordinates of a sample of 253 IRAS sources. Results. We have identified 103 sources as post-AGB stars, 21 as "transition sources", and 36 as planetary nebulae, some of them strongly reddened. Among the rest of sources in the sample, we were also able to identify 38 young stellar objects, 5 peculiar stars, and 2 Seyfert galaxies. Up to 49 sources in our spectroscopic sample do not show any optical counterpart, and most of them are suggested to be heavily obscured post-AGB stars, rapidly evolving on their way to becoming planetary nebulae.Conclusions. An analysis of the galactic distribution of the sources identified as evolved stars in the sample is presented together with a study of the distribution of these stars in the IRAS two-colour diagram. Finally, the spectral type distribution and other properties of the sources identified as post-AGB in this spectroscopic survey are discussed in the framework of stellar evolution.

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Post‐asymptotic giant branch (post‐AGB) stars are key objects for the study of the dramatic morphological changes of low‐ to intermediate‐mass stars on their evolution from the AGB towards the planetary nebula stage. There is growing evidence that binary interaction processes may very well have a determining role in the shaping process of many objects, but so far direct evidence is still weak. We aim at a systematic study of the dust distribution around a large sample of post‐AGB stars as a probe of the symmetry breaking in the nebulae around these systems. We used imaging in the mid‐infrared to study the inner part of these evolved stars to probe direct emission from dusty structures in the core of post‐AGB stars in order to better understand their shaping mechanisms. We imaged a sample of 93 evolved stars and nebulae in the mid‐infrared using VLT spectrometer and imager for the mid‐infrared (VISIR)/VLT, T‐Recs/Gemini‐South and Michelle/Gemini‐North. We found that all the proto‐planetary nebulae we resolved show a clear departure from spherical symmetry. 59 out of the 93 observed targets appear to be non‐resolved. The resolved targets can be divided into two categories. (i) The nebulae with a dense central core, that are either bipolar and multipolar and (ii) the nebulae with no central core, with an elliptical morphology. The dense central torus observed likely hosts binary systems which triggered fast outflows that shaped the nebulae.