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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.

Abstract. We present the near infrared photometry of a new sample of 225 IRAS sources, many of them previously unidentified in the literature, selected because their far infrared colours are similar to those shown by known planetary nebulae. The results obtained are used to establish the main source of near infrared emission. Combining this information with the far infrared IRAS data and a few additional criteria we determine the nature and evolutionary stage of all the sources observed so far, including those for which near infrared photometry was previously reported in Papers I and II.Among the unidentified IRAS sources in our sample we find only a small percentage of planetary nebulae, many of them very young and dusty, showing peculiar near infrared colours. Most of the new objects observed in the near infrared are identified as transition objects in the previous stages of the stellar evolution. Among them, we find heavily obscured late-AGB stars, early post-AGB stars still obscured by thick circumstellar envelopes which are probably the true progenitors of planetary nebulae, and a significant fraction of stars with bright optical counterparts showing little or no near infrared excess, which we associate with highly evolved post-AGB stars with low mass progenitors, which may never become planetary nebulae. In addition, we also find a small percentage of young stellar objects, as well as a few Seyfert galaxies.We conclude that, in most cases, based on near infrared data alone, it is not possible to give a confident classification of the unidentified IRAS source. However, the near infrared is shown to be a powerful tool, specially when dealing with objects which are heavily obscured in Send offprint requests to: P. García-Lario Based on observations collected at the European Southern Observatory, La Silla (Chile) and at the Spanish Observatorio del Teide, Tenerife, Spain. Table 6 is only available electronically at the CDS via anonymous ftp to cdsarc.u-strasbg.fr (130.79.128.5) or via http: //cdsweb.u-strasbg.fr/Abstract.html the optical. In this case, the detection of the near infrared counterpart is the only way in which we can extend the study of these sources to other spectral ranges and may be crucial to understand the short-lived phase which precedes the formation of a new planetary nebula.

A summary is given of planetary nebulae abundances from ISO measurements. It is shown that these nebulae show abundance gradients (with galactocentric distance), which in the case of neon, argon, sulfur and oxygen (with four exceptions) are the same as HII regions and early type star abundance gradients. The abundance of these elements predicted from these gradients at the distance of the Sun from the center are exactly the solar abundance. Sulfur is the exception to this; the reason for this is discussed. The higher solar neon abundance is confirmed; this is discussed in terms of the results of helioseismology. Evidence is presented for oxygen destruction via ON cycling having occurred in the progenitors of four planetary nebulae with bilobal structure. These progenitor stars had a high mass, probably greater than 5 M . This is deduced from the high values of He/H and N/H found in these nebulae. Formation of nitrogen, helium and carbon are discussed. The high mass progenitors which showed oxygen destruction are shown to have probably destroyed carbon as well. This is probably the result of hot bottom burning.Key words. ISM: abundances -planetary nebulae: general -evolution -HII regions -Sun: abundances -stars: abundances IntroductionPlanetary nebulae (hereafter PNe) are an advanced stage of stellar evolution of low and intermediate mass stars. Their abundances have been changed by various processes which have occurred in these objects since their formation. Not all elements have been affected in the same way or to the same extent; some elements have not been affected in the course of evolution. Our present purpose is to determine which elements have been processed in the course of evolution, and which are the affected nebulae. This is done using the abundances determined with the help of the Infrared Space Observatory observations (ISO, Kessler et al. 1996). In particular the result of the Short and Long Wavelength Spectrometers have been used (hereafter SWS and LWS respectively).In order to determine which abundances have changed in the course of evolution it is necessary to find the initial abundances before evolution began. These values are taken from three sources: HII regions, young (B and O type) stars, and the Sun. For the first two groups it is known that they show a gradient in abundance as a function of their distance from the Galactic Center (see for example Shaver et al. 1983;and Rolleston et al. 2000). Thus in comparing planetary nebulae abundances with that in HII regions and young stars this gradient must be taken into account. This is done by plotting the abundances as function of the distance from the galactic center and then comparing these plots.Two potential problems arise. Firstly, the distance to individual objects from the Sun is often uncertain. This is especially a Based on observations with ISO, an ESA project with instruments funded by ESA Member States (especially the PI countries: France, Germany, the Netherlands and the United Kingdom) and with the participation of ISAS an...