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1. Introduction

The symbiotic binary systems possess dense gaseous nebulae formed, in most cases, by the wind of their cool components. At some stages of their evolution, however, these nebulae are formed by the winds of the two components. The system AG Peg (HD 207757) is the oldest known symbiotic nova. It has undergone a single outburst (Lundmark 1921; Boyarchuk 1967), which is the most prolonged one among the outbursts of symbiotics. Its visual light was 9tex2html_wrap_inline1330 before the middle of the last century. Between the years 1841 and 1855 it has begun to increase, reaching a maximum of approximately 6tex2html_wrap_inline1330 around 1885. Then it has gradually decreased and now it is practically equal to its value before the outburst.

The analysis of the observations carried out in different spectral regions during the past years, imply one and the same model of this system - a stage of colliding winds. For the first time this model was proposed by Penston & Allen (1985) on the basis of three IUE high resolution spectra, where a high velocity P Cyg wind was detected in the lines of NV, NIV, CIV and HeII. Since the cool giant also loses mass through a stellar wind, it was concluded that the two winds probably interact.

Later the UV spectrum of AG Peg was studied by Kenyon et al. (1993), Vogel & Nussbaumer (1994) and Altamore & Cassatella (1997). Each of these analyses showed that a hot high velocity wind is observed in the lines of NV, CIV and HeII and in every one of them a conclusion about colliding winds was made.

A model of winds in collision was proposed also by Tomov (1993b) on the basis of profiles, fluxes and radial velocities, derived from homogeneous high dispersion spectral observations in the visual during two consecutive orbital cycles (Tomov & Tomova 1992; Tomov 1993a). The broad components of the first Balmer lines and the line HeII 4686 were caused by radial flow of gas with high velocity.

Supposing a regime of colliding winds in the AG Peg system Vogel & Nussbaumer (1994) theoretically calculated the radio image of its nebula being in good agreement with the observed one. A proof for existence of a shock region of the winds was obtained on the basis of ROSAT data by Mürset et al. (1995). They created a hydrodynamical model whose X-ray energy distribution was in agreement with the observations.

There are also observational data showing that some parameters of the wind of the hot companion are changed. According to both UV (Kenyon et al. 1993; Vogel & Nussbaumer 1994; Altamore & Cassatella 1997) and visual (Hutchings & Redman 1972; Ilmas 1987; Tomov & Tomova 1992) observations, the lines, appearing in this wind, began to weaken after the year 1978. The decrease of their intensities is determined by decreasing mass-loss rate (Kenyon et al. 1993; Vogel & Nussbaumer 1994; Altamore & Cassatella 1997). Having in mind their behaviour Zamanov & Tomov (1995) prognosticated that in the near future the change of the mass-loss at the observed rates will cause its decreasing below a given minimal value. At that time the momentum of the hot wind will not be sufficient for realizing a collision and the system will move to a regime of a wind accretion. This transformation will be the final stage of the outburst begun in the first half of the last century.

The change of the mass-loss rate of the hot companion causes changes in the dynamics and the physical parameters of the nebular envelope and consequently of its spectrum. That is why we observed the blue region where most of the visual emission lines are situated. In this paper we analyse and discuss intermediate resolution observations of AG Peg taken in 1995 and compare them with the observations in the middle of the last decade (Tomov & Tomova 1992; Tomov 1993a) to study the evolution of the visual emission lines during the final stage of the outburst.


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