4 Discussion

The timescale of some of the variations is probably very short, one month at least (July and August 1990) according to our observations. Moreover, taking into account Munari's et al. photoelectric observations performed two weeks before our spectroscopic observations of July/90, variations of $\Delta U = 0.17$ mag, $\Delta B = 0.09$ mag, $\Delta V = 0.05$ mag, $\Delta R = 0.02$ mag and $\Delta I = 0.01$ mag occur from one night to the next one, tending to increase the photometric gradient with decreasing wavelength and can thus be associated with the hot component. These rapid photometric variations or "flickering" effect are observed in other symbiotic systems with low hot component luminosities and it is usually interpreted as some measure of fluctuations in the accretion rate (Dobrzycka et al. 1996).

On the other hand, there are variations that seem to take place in Hen 1761 in a longer timescale. [OIII]$\lambda$5007 Å was present on the spectra obtained in 1988, it disappeared in 1989 and later since 1990, together with the other high-excitation emission line HeII$\lambda$4686 Å, displayed important changes in the relative fluxes, reaching a minimun in 1991 and then, increasing until 1995. If we assume that the [OIII] lines arise from an homogeneous nebula with $T_{\rm e}=10\,000$ K and electron density of $n_{\rm e} = 10^{7} - 10^{8}\,{\rm cm}^{-3}$, the size of the [OIII] emitting region can be calculated (Mikolajewska 1985). For a mean unreddened flux of the $\lambda$5007 line, $F(5007) = 1.85\,10^{-12}$ erg cm^-2 s^-1, and adopting the distance d = 2.2 kpc, we obtain a range for the radius of the nebula of 4 to 7 AU. If a typical binary separation of 2 AU for S-type symbiotic stars (Kenyon 1986; Schmid 1995) is considered, the [OIII] lines are not produced close to the hot component and consequently, their radial velocity variations would hardly correspond to the orbital motion.

When the cool component displays the earlier spectral type, M1.0, an additional blue continuum, coming from the hot source is veiling the M spectrum. In the blue region, metallic lines are in absorption; the forbidden lines are weaker; the high ionization emission lines are absent and the FeII emission lines are stronger. A considerable weakening in the blue continuum is observed since July 1992. The TiO absorption band depths became greater and consequently the spectral type of the giant gets increasingly later than the former observations. During the latest observations the forbidden lines requiring still higher excitation conditions (e.g. [FeVI], [FeVII] and [CaV]) are detected for first time in Hen 1761, some of them with a double-peaked profile as has been observed in other symbiotic stars. The FeII emission lines show a systematic decrease of intensities and almost disappear in 1995.

Strong central absorptions are visible in the H lines particularly H$\alpha$ and H$\beta$ with variations in time. Many authors have suggested that these profiles are originated in an accretion disc which surrounds the hot component of the system.

The absence of the Raman scattered OVI emission lines $\lambda\lambda$ 6825, 7082 Å deserves to be mentioned. These lines are almost always present in symbiotic systems with high excitation lines, such as [FeVII] or [NeV] (Schmid 1996) but according to Kenyon (1986) the symbiotics that have noticeable [FeVII] emissions and no trace of $\lambda\lambda$ 6825, 7082 Å, are likely candidates for an accreting main sequence star as the hot source.