One of the defining characteristics of symbiotic stars is the presence of nebular emission lines, which indicates the existence of a circumstellar nebula. Since the excitation and ionization of atomic emission lines depend on the temperature and density of the emission region, line observations (optical or ultraviolet) therefore only selectively probe part of the circumstellar nebule. Radio observations being less sensitive to “hot spots”, reflect the global character of the ionized region and are more suitable for determining the density structure of the nebula. Recent advances in the technique of aperture synthesis allow observations to be made with high resolution (˜0ʺ.1) and direct mapping of nebular structure is now a possibility. Since the circumstellar nebula is most likely to have originated from the stellar components of the symbiotic system, a better understanding of the nebula may provide important clues to the nature of symbiotic stars.

Infrared photometry and spectroscopy of symbiotic stars is reviewed. It is shown that at wavelengths beyond lym these systems are generally dominated by the cool star’s photosphere and, indeed, are indistinguishable from ordinary late-type giants. About 25% of symbiotic stars exhibit additional emission due to circumstellar dust. Most of the dusty systems probably involve Mira variables, the dust forming in the atmospheres of the Miras. In a few cases the dust is much cooler and the cool component hotter; the dust must then form in distant gas shielded from the hot component, perhaps by an acccretion disk.

Spectroscopy at 2μm can be used to spectral type the cool components, even in the presence of some dust emission. Distances may thereby be estimated, though with some uncertainty.

Spectroscopy at longer wavelengths reveals information about the dust itself. In most cases this dust appears to include silicate grains, which form in the oxygen-rich envelope of an M star. In the case of HD 330036, however, different emission features are found which suggest a carbon-rich environment.

It is very well known that symbiotic stars are displaying at the same time spectral characteristics of hot stars, cool stars (principally of M type), nebulae and circumstellar dust envelopes.

The investigation of the near IR is particularly interesting because one finds there important features of these different objects.

Relatively few spectra of symbiotic stars have been recently published in the photographic infrared. We have observed six objects during the period 1962-1977 with a grating spectrograph attached to the newtonian focus of the 120-cm telescope at Observatoire de Haute Provence. The reciprocal dispersion is 230 A.mm-1 and the region 5800 to 8800 A has been covered using hypersensitized IN plates. The minimum equivalent width for an emission line to be seen is about 0.5 A. The spectra are displayed on plates I and II. We now briefly review the main spectral characteristics.

The traditional definition of Symbiotic Stars (SS) is that of objects which display a combination spectrum (e.g. Merrill, 1950) that is emission lines requiring high-excitation conditions, superposed to the continuum and absorption features of a low-temperature star, most commonly an M-type giant. About one hundred of SS are known and listed today. It is anyway apparent that the classification criteria are rather rough, and since the excitation varies from the simple Me to SS with coronal emission, it is not well defined where a clear division should be made. As a result, the available lists include a very heterogeneous set of objects, probably different phases in stellar evolution. Moreover it has been remarked that SS show a rather confusing variety in their spectroscopic and photometric behaviour. Different intensities of both absorption and emission lines are reported from star to star. These facts indicate a high degree of individuality among SS, which is partly cause and/or effect of the adopted definitions.

In connection with the discussion of the individual symbiotic stars, we present low resolution, flux calibrated spectra and high resolution H α profiles for each of 12 objects.

Spectra of symbiotic stars have been obtained with the Haute Provence Multiplhot detector system. This has 512 pixels, so one can study either a large spectral region at low resolution, or a smaller region with a higher resolution.

General characteristics of emission line spectra from symbiotic stars are outlined. Data from some special line ratios in the 1000 Å - 3000 Å range, and others connecting the visual and the far UV lines are presented, and their application to symbiotic stars is discussed. Integrated fractional abundances for ions easily observed in the far UV are given to facilitate abundance determinations for nebular conditions. It is found that the physical conditions of the regions emitting the emission line spectra differ considerably among different symbiotic stars.

Prior to the launch of the IDE satellite in early 1978, the only symbiotic star previously detected in the ultraviolet by earlier UV satellites, such as the 0A0-2, TD-1 and ANS experiments, was AG Pegasi = HD 207757 (Gallager et al. 1979). These broad-band observations indicated that the symbiotics as a class may show a significant ultraviolet flux and thus they became natural candidates for a survey with the IUE satellite. The following is an interim report on a survey of the symbiotics, both at low and, for AG Pegasi and CH Cygni, at high resolution.

Observations are reported of 19 symbiotic stars made with the imaging proportional counter of the Einstein Observatory. Three of the objects (HM Sge, V 1016 Cyg and RR Tel) were detected as soft X-ray sources. All three have shown slow-nova eruptions in the past 40 years. The data are interpreted as support of a model for slow novae involving thermonuclear events on white dwarfs which accrete from M giant companions. Symbiotic stars in their steady state, not being detected X-ray sources, are presumed to be powered by the accretion process alone.

The symbiotic star AG Draconis was observed by the HEAO-2 Imaging Proportional Counter (IPC) and found to be an unusually intense source of very soft X-rays.

The spectrum of CH Cygni usually has the appearance of a normal M6 III star, but at intervals of several years, it has phases of activity during which it shows the characteristics of a symbiotic star, that is, a blue continuum extending shortward of the Balmer discontinuity, and strong emission lines of He I, Fe II, [Fe II] , [S II], [0 1] , [0 III]; the Balmer lines and the H and K lines of Ca II present emission wings. Outbursts were observed: 1) in September 1963; the star returned to its normal phase by August 1965 (Deutsch, 1964); 2) in June 1967 (Deutsch, 1967); the spectrum was observed by Faraggiana and Hack (1971) until December 1970, when the activity phase was over; 3) another outburst was observed in August 1977 (Morris, 1977; Fehrenbach, 1977), and at the time of our last spectroscopic observations (June 1981) it was still going on. During this last period of activity several observations were made with IUE in both the low and high resolution modes, from 1175 A to 3100 A.

CH Cyg has exhibited a great deal of activity in the recent times. We obtained two spectra with dispersion of 9 A/mm with the 2 m telescope of the Ondrejov Observatory on the nights of July 4/5 and August 6/7, 1981. The differences between these spectra are conspicuous. The description of the spectral changes during this period follows:

The Balmer lines became broader, the R wing of the early Balmer shell lines being partly suppressed by absorption. The inverse P Cygni profiles became apparent from H8 to H30.