5 Conclusions

We summarize our study by saying that Hen 1761 is an S-type symbiotic star with important spectral variations. Some of the spectral changes can be explained if we consider that Hen 1761 has to go through outburst and quiescence stages, in the case that the system contains a red giant star M5 and a hot white dwarf with an accretion disk, which powers the activity of the system. Unfortunately, it is not possible to specify both stages due to the lack of a light curve following the spectral changes. During the activity in 1990-91 the spectrum shows the characteristic outburst properties as is somtimes observed in symbiotic stars: the presence of HI, HeI, HeII, NIII and FeII emission lines and forbidden lines of [FeII], [NeIII] and [OIII]. Absorption lines like the doublet NaI and CaI $\lambda$4226 Å are also present in this phase. On the other hand, a blue continuum veils the giant's features and an earlier spectral type for the cool component is inferred from the optical spectrum. The quiescence phase may have started in 1992, when the FeII emission lines decreased, the forbidden lines of low-excitation [FeII] and [NeIII] disappeared and forbidden lines requiring still higher excitation conditions (e.g. [FeVI], [FeVII] and [CaV]) are observed. At the same time, the veiling of the absorption bands on the blue continuum decreases substantially. The [OIII] emission lines are present as much as the HeII ones, during both phases but their intensities are increasingly noticeable on our quiescent observations. The variability of the HeI intensity ratios presented in Fig. 5 suggests that the outburst produces changes in the density of the nebula where these lines are formed.

Other spectral variations are observed in Hen 1761 due perhaps to orbital motion or associated with the flickering activity. In order to separate the causes of the variations, a series of photometric and spectroscopic observations are necessary. Thus, it would be very important to analyse the UV spectrum of Hen 1761 with the aim of learning more about the nature of the hot component and confirming the existence of an accretion disk.

Acknowledgements

The authors wish to thank Prof. J. Sahade for careful reading of the manuscript. The authors also acknowledge the use of the CCD and data acquisition system supported under U.S. National Science Foundation grant AST-90-15827 to R.M. Rich.