Introduction

Initial spectroscopic observations of symbiotic stars revealed them to be peculiar beasts which somehow combine the absorption features of a red giant with the emission lines of a planetary nebula (Figure 1.1). Analysis of their photometric behavior led to additional confusion, as long, apparently quiescent, intervals would be rudely interrupted by 2-3 mag outbursts (Figure 1.2). While the eruptions have captured the imagination of many researchers, a few groups recognized the value of acquiring photometric and spectroscopic data during the lengthy quiescent phases. These historical data, briefly summarized in the Appendix, clarify the long-term behavior of these systems, and provide strong support for the binary models proposed by Berman, Hogg, and Kuiper.

The field of symbiotic stars has grown tremendously in the past twenty years, and data covering a significant portion of the electromagnetic spectrum have been acquired for a number of systems. This information allows definitive tests of the basic models developed in the 1930's and 1940's, and generally supports the basic binary picture discussed in Chapter 2. New results from infrared, optical, and radio photometry demonstrate that symbiotic stars are binary systems (P ∼ 200 days to > 20 years) composed of a late-type giant star and a hot component surrounded by an ionized nebula. Detailed spectroscopic observations have resulted in refined portraits of individual systems, which show a wonderful variety of accreting hot components and mass-losing red giants.