In 1975 Webster and Allen (1975) divided all symbiotic stars into two groups-those in which the 1-4μm continuum show only the presence of a cool star (type S),and those in which dust emission dominates (type D). With the exception of some of yellow symbiotic stars, the dust presence in others correlates with the spectral type of their cool components. That is why one can say that S-type symbiotics contain red giants with spectral type earlier than M6-M7.
At the IAU Colloq. N 7Q Allen (1982) noted that it is difficult to escape the conclusion that symbiotic stars contain normal cool giants. Nowadays it is certaiu to be correct because the modern observations of S-type symbiotic stars have not yet discovered any specific distinctions between their cool stellar components and normal red giants. At the same time it should be noted that some of these, for example, Z And, CI Cyg may be interacting binaries in which the cool component apparently fields its Roche lobe and unstable accretion of gas from the red giant onto its hot companion leads to the out bursts of the latter (Kenyon and Webbink 1984; Yudin 1987).