There are many emission-line objects classified as planetary nebulae (PN) which are of stellar appearance. Assuming that this classification is correct, such objects have to be either young PN with very small nebulae, or old PN with large nebulae, which are very faint and so far invisible. It would be very important from the evolutionary point of view to differentiate between these two possibilities.
Besides, there are 43 emission-line objects which appear simultaneously in the catalogues of planetary nebulae as well as in the catalogues of symbiotic stars (SS; see Allen 1984). The variability and the presence of a red component do not seem to be sufficient criteria for distinguishing both cases. In spite of the fact that the origin and the early evolution of PN is little known, the knowledge of the existence of nebulae can help to classify such objects better than in the past.
This is the first series of objects the choice of which (Table 1 (click here)) was made mainly with regard to the above circumstances.
