2 Classification criteria

The optical spectra of symbiotic stars are characterized by the presence of absorption features and continuum, as appropriate for a late-type M giant (often a Mira or semi-regular, SR, variable), and strong nebular emission lines of Balmer H I, He II and forbidden lines of [O III], [Ne III], [Ne V] and [Fe VII]. Some symbiotics - the yellow symbiotic stars - contain K (or even G) giants or bright giants. The spectra of many symbiotic systems also show two broad emission features at $\lambda\, 6825$ Å and $\lambda\, 7082$ Å. These features have never been observed in any other astrophysical objects -- only symbiotic stars with high-excitation nebulae. For many years there was no plausible identification for these lines, but Schmid 1989 pointed out that the $\lambda\lambda$ 6825, 7082 lines are due to Raman scattering of the O VI $\lambda\lambda$ 1032, 1038 resonance lines by neutral hydrogen.

To classify an object as symbiotic star we adopted the following criteria:

1.

The presence of the absorption features of a late-type giant; in practice, these include (amongst others) TiO, $\rm H_2O$, CO, CN and VO bands, as well as Ca I, Ca II, Fe I and Na I absorption lines;

2.

The presence of strong emission lines of H I and He I and either

• emission lines of ions with an ionization potential of at least 35 eV (e.g. [O III]), or
• an A- or F-type continuum with additional shell absorption lines from H I, He I, and singly-ionized metals.

The latter corresponds to the appearance of a symbiotic star in outburst;

3.

The presence of the $\lambda$ 6825 emission feature, even if no features of the cool star (e.g. TiO bands) are found.

Our adopted criteria represent a compromise: a collection of the classification criteria proposed in the past 70 years (see Kenyon 1986 for details), based on the examples of well-studied and widely accepted symbiotic objects. We believe that such an approach is appropriate, especially given that symbiotic stars are variables with timescales often exceeding a dozen years and that -- as Kenyon very sensibly noted -- "every known symbiotic star has, at one time or another, violated all the classification criteria invented''. For those who would prefer additional or different definitions, we give the highest ionization potential ever observed in the optical and UV (for objects that have been observed at least once with the International Ultraviolet Explorer -- IUE). We also comment on all objects for which our classification may not seem obvious (e.g. V934 Her, which some readers may consider to be symbiotic, but which in our catalogue is classified as a suspected symbiotic star).