The IRAS Low Resolution Spectrometer detected a small number of sources
 with featureless spectra of very low colour temperatures (about 200 K)
 which are not HII regions. The nature of these objects is unknown.
 Subsequent CO observations of some of these sources showed that their CO
 properties are typical of mass-losing AGB stars.  The circumstellar dust
 shells of these objects are unusual because of the low temperature and
 because no features are seen.  The broad-band IRAS colours of these
 sources are similar to those of planetary nebulae, so they may be PNs
 or objects evolving from the AGB to the PN phase.

 In order to identify the nature of these sources we request ISO
 SWS observations, and for a few brighter objects, with the LWS.
 Such observations would (a) provide us with an accurate picture of the
 spectral energy distribution of these sources which radiate dominantly in
 the 15 to 45 micron wavelength range, and (b) open the possibility that
 dust features or emission lines would be detected.  It is also possible
 that some underlying stellar features might be detected at short
 wavelengths.  The emission from these objects has so far been detected
 only in the far-infrared and therefore the identification of the nature
 of these sources can only be done in the 25 micron wavelength range with
 ISO.

 The featureless nature of the LRS spectra might be explained if the dust
 were graphitic in nature, or it may be an entirely different type of dust
 such as that which produces the featureless LRS spectra for M-type stars.
 With these possibilities in mind it is important to obtain LWS data for
 the brighter of these objects to search for longer wavelength dust
 features which may allow identification of the type of dust in the event
 that no features are seen at shorter wavelengths.