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More than a dozen classical novae in outburst have been observed with the International Ultraviolet Explorer since its launch in 1978. These data represent a crucial breakthrough in understanding several aspects of the classical novae phenomenon including nucleosynthesis, the energetics of the explosion, the dynamics of the mass ejection, and the processes of dust formation. Also they are important to gain insight on the nature of the white dwarf components and then on the evolutionary scenario of these binary systems.The UV emission lines include important transitions from several species such as He, C, N and O, Ne, Mg, and Al which, whenever symultaneous optical data were available, have lead to a detailed knowledge of the physical conditions in the nebular phases and to the determination, with unprecedented accuracy, of the chemical abundances in the ejecta (see e.g.: Stickland et al. 1981; Williams et al. 1985; Snijders et al. 1987). Precise abundance determinations provide crucial constraints on nucleosinthesis during TNR as well as on the efficiency of diffusion and mixing processes. The ultraviolet range provides also effective means to determine the value of the reddening from the strength of the 2200 A dust feature or from the emission line ratios; an accurate determination of this parameter from the UV is essential, for example, to determine the energy budget of novae and the temperature of the remnants.

About 16 old novae are accessible to IUE, but only five (RR Pic, V 603 Aql, HR Del, DQ Her, and GK Per) have been observed in the ultraviolet and studied in any detail. Considering that, because of selection effects, only few and the brightest objects have been observed, it not possible to draw general conclusions on the UV luminosity LUVand mass accretion rates Ṁaccof old novae. We plan to improve the rather poor statistic by observing in the near future all objects falling above the IUE detection limit of m(V) ≃ 15 (i.e. X Ser, DK Lac, DN Gem, and HR Lyr). In the following we present preliminary results for 7 old novae observed with the IUE satellite whitin our observing programmes or obtained from the IUE archive.

Spectra of the old nova V603 Aql taken with the HST-GHRS did not reveal resonance doublet absorption components arising in material ejected in the 1918 outburst. However absorption components, probably produced by a disk wind, were observed, though most line emission probably comes from regions associated with an accretion disk. Most strikingly, dramatic line profile variations on time-scales of not much more than 10 m were seen.

We present results for IUE, optical and IR observations of Nova Muscae 1983, from early outburst to January 1986 obtained by the European IUE Target of Opportunity Team. A detailed description of the data will appear elsewhere (Hassall et al., 1989), but here we summarise the most important results.The outburst lightcurve initially indicated a fast speed class for this nova, but was later characterised by a rather slow optical decline with two or more secondary outbursts with sudden doubling of the bolometric flux. In Figure 1, we show the contributions of X-ray, UV, optical and IR to the total luminosity for 1200 days following outburst, assuming a distance of 4.3kpc and an interstellar extinction E(B-V)=0.5. In the absence of dust formation, first the UV and later the X-ray flux (Ögelman et al, 1984) dominate the radiative energy late into the nebular phase. There was a plateau stage lasting about 500 days, with a bolometric luminosity of ~ 1038ergs s−1 near the Eddington limit. The secondary outbursts were thus super-Eddington. We estimate a total outburst energy (including kinetic and gravitational potential energy of the ejecta) of ~ 5·l046 ergs, corresponding to a mass of ~ 4·10−6Mº of hydrogen burnt in the thermonuclear runaway.

Z And is considered as the “prototype” of the symbiotic stars. Besides its symbiotic spectrum, the star is also known for its characteristic light curve (and for the related spectral variations). Since many theoretical speculations on Z And and similar objects have been based on the luminosity and spectral variations of this star, it is important to make a critical analysis of the observational data concerning it.

Z Andromedae, often considered as the prototype of symbiotic stars, experimented, after several years of quiescence, a small outburst in March-April 1984, followed by a larger one in September-October 1985. The imminence of a new activity phase was predicted by Viotti et al. (1982). Z And is, together with AG Draconis, the only symbiotic star observed both during quiescence and activity with the IUE satellite. The early photometric and spectroscopic history of Z And has been recently reviewed by Kenyon (1986).The behaviour of Z And in the ultraviolet during quiescence has been studied by Fernåndez-Castro et al. (1988), on the basis of data obtained by the International Ultraviolet Explorer from 1978 to 1982. In that period of time, the UV continuum and the emission line fluxes varied quasiperidically with a period of about two years, in phase with the Hα variability found by Altamore et al. (1979), with the UBV photometry by Belyakina (1985) and also in agreement with the ephemeris given by Kenyon and Webbink (1984). The electron density derived from Si III/C III] flux ratio, also varied in phase with the UV flux. In particular, a correlation was found between the electron density variations, and the UV continuum flux at 2900 A. This fact, together with the presence of a Balmer jump in emission, indicates that the principal contributor at those wavelengths is nebular emission, mainly free-bound transitions.

S22 is a peculiar supergiant in the Large Magellanic Cloud whose optical spectrum is characterized by numerous emission lines including Fell and [Fell] (Muratorio 1978). A detailed study performed by Friedjung and Muratorio (1980) led to two main conclusions: (a) A wind with a velocity of 70 km s-1 and a mass loss rate between 4x10-6 and 5x10-5 Mʘ yr-1 is suggested by the examination of the Balmer line P Cygni profiles, (b) The Fell and [Fell] emission lines come from a different region which is optically very thick in the permitted lines. Emission line curve-of-growth methods indicated that the region of line formation has a surface area perpendicular to the line of sight equal to that of a disk perpendicular to this direction with a radius between 3x1013 and 3x1014cm.

IUE observations at high and low resolution of Nova Aql 1918 show neither evidence of outflow nor the presence of nebular lines. This indicates that the shell ejected at the time of the outburst and surrounding the system for many years (Mustel and Boyarchuck, 1970) has by now disappeared.The high excitation spectrum presents rapid variations in the far UV and eclipse effects in the near UV that seem well correlated with the orbital phase.The observations can be interpreted in terms of phenomena occurring in or near the accretion disk surrounding the white dwarf. However, the small inclination of the orbital axis raises serious problems in trying to model the system.

The formation of a new planetary nebula should be a rare event, and the case of V1016 Cyg may represent such an example. This is the reason of the considerable interest of the astronomers in this star since the discovery of its outburst in 1965.

The standard classification criteria, when applied to peculiar objects, such as emission line and shell stars, cannot easily be interpreted in terms of their intrinsic physical characteristics. Observations outside the optical spectral region may provide complementary information on their nature leading to a better spectral classification.

Coordinated IUE and EXOSAT observations of the symbiotic Mira R Aqr, made in June and December 1985, at phases 0.0 and 0.5 of the Mira light curve, are discussed. A weak X-ray flux has been for the first time unambiguously detected without any significant change with the Mira light curve. Also the UV spectrum of the central object has not varied, while the presence of high ionization lines (NV and HeII) in the spectrum of the jet-like feature is confirmed. The data are discussed in the light of a model in which the jet is preexisting material illuminated by the recently increased X and UV radiation from the inner edge of the accretion disk around the degenerate companion of the Mira primary.

The galactic LBV object AG Car is unique for assembling several interesting features: (i) a light curve typical of Hubble-Sandage variables, (ii) a largely variable P Cygni-type spectrum, (iii) a strong IR excess, and especially (iv) a smallring nebula. Caputo and Viotti (1970) found that during 1949-59 the AG Car spectrum varied betweenA1andB0equivalent spectral types. A dramatic evolution occurred in recent years, when the visual magnitude gradually faded from V=6 in 1981 to V=8 in 1985, followed by a rise of the line excitation fromAeqin 1981 (Wolf and Stahl 1982), toBeqin 1983 (Viotti et al. 1984), and to0fpe/WN9in January 1985 (Stahl 1986). This behaviour has represented a unique chance to follow a LBV in different spectral ranges during its major photometric and spectral changes. In the following we present the results of a long term monitoring of the star and of its nebula in the optical and UV. We propose thatAG Car evolved in recent times from a red supergiant phase, and that the present LBV phase is characterized by acritical structureof the stellar atmosphere.

AG Car is one of the few Luminous Blue Variables (LBV’s) in our galaxy. The star is a photometric variable which had a recent outburst in 1981 (Whitelock et al., 1983; Wolf and Stahl, 1982) and is surrounded by a resolved ring nebula (Thackeray, 1950; Paresce, 1989). These properties indicate that AG Car is a typical LBV. However, at its canonical distance of 2-2.5 kpc, derived from its assumed membership of the Car OBI or OB2 association, AG Car has a bolometric luminosity of Mbol⋍ −8.5 to −9.0 and is located significantly below the Humphreys-Davidson limit, and also below the upperlimit for red supergiants (RSG). This implies that AG Car might have been a RSG and that the ring might be due to the interaction of the RSG wind and the LBV wind.

In this paper we provide preliminary results of multifrequency observations of BF Cyg carried out in July 1986. The ultraviolet spectra were obtained on July 26, 1986 using the IUE satellite. The optical observations were made at the Observatorio del Roque de los Muchachos (La Palma, Canary Islands) in July 1986 during the night 13/14 using the Isaac Newton 2.5m telescope with the Intermediate Dispersion Spectrograph (IDS, 500 mm camera) and the Image Photon Counting System (IPCS). The infrared observations were made during the night 13/14 of July, 1986, at the Observatorio del Teide (Tenerife, Canary Islands) using the Carlos Sanchez 1.5m telescope and an infrared single-channel photometer with an InSb detector.

High and low resolution IUE spectra of CI Cyg were obtained at VTLSPA during 1979-81 allowing for an analysis of the spectral variations related to the decreasing activity of the star, and to the eclipse (June 1980).In the high resolution spectra the emission lines have a width slightly larger than the instrumental one. This fact is particularly evident in the HeII 1640 A line and could be related to the peculiar behaviour of this line at low resolution as reported by Michalitsianos et al. (this volume). A systematic radial velocity difference between permitted and intercombination lines was found; this difference should be connected with the structure of the emitting region(s). “Secular” and eclipse varia tion was found in particular in the intercombination line intensities (Viotti et al. 1980). An electron density of ∼ 0.3–1.5×1010 cm–3 was evaluated from the intensity ratios of the NIII] emission lines. No significant difference in these ratios and in the CIII]/NIII] ratio as well was found,during 1979, 1980 (eclipse) and 1981 suggesting no large Ne variation with both the activity phase and eclipse of the star. This result should imply a low density gradient in the partially eclipsed NIII] and CIII] regions. A more detailed analysis of the high resolution data is in course to better clearfy these points, and their implications on the possible models for CI Cyg.

The ultraviolet spectrum of RR Tel was extensively studied with the IUE satellite since 1978 in both the high and low resolution modes. A comprehensive study of these observations was made by Penston et al.(l98l) who measured more than 400 emission lines. As it is clearly shown in figure 1, the UV spectrum of this symbiotic star is very rich in emission lines. Like in the optical spectrum (Thackeray 1977), the UV spectrum presents emission lines of ions belonging to a wide range of ionization energies, from neutral up to four and five times ionized species (OV, MgV, CaVI, etc.). Permitted, intercombination and forbidden transitions were found which may allow a diagnosis of the physical conditions of the emitting regions. Electron temperatures of 1.2-1.9 104°K and densities of 106-108 cm-3 were derived by Penston et al.

The high resolution ultraviolet spectrum of AG Dra was observed with IUE in April and August 1981 at phases 0.50 and O.69 according to the Meinunger (1979, Inf. Bull. Var. Stars No.1611) U-light curve. The UV spectrum of the star appears rather different from that of the other classical symbiotic stars. The low resolution IUE spectrum of AG Dra shown in figure 1. The continuum is rather strong with respect to the emission lines and detectable at high resolution. Many intense interstellar lines are present, in spite of the low reddening of the star (E(B-V)=0.06, according to the depth of the 2200A interstellar band).