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6 Discussion

The presented spectroscopic behaviour largely resembles other RNe in different phases. In the early phases of outburst broad emission lines dominated the spectra. The extensive wings reached ${\rm FWZI} \sim 8000-9000$ km s-1 (Figs. 1 and  3). Later they became narrower. The low-resolution spectra covering the whole optical range show broad emission complexes $\lambda\lambda$4500-4700, $\lambda\lambda$4900-5100, $\lambda\lambda$5600-5800, $\lambda\lambda$5800-6000. Very similar spectra were obtained, e.g., for V394 CrA 5 and 6 days after maximum (Sekiguchi et al. 1989a; Williams et al. 1991; Williams 1992) and for U Sco at +1.45 days (Anupama & Dewangan 2000) and between +3 and +5 days (Munari et al. 1999). Interestingly, Munari et al. (1999) observed exactly the same change of He II lines at 8237 Å and 4686 Å in U Sco between +1.63 to +4.59 days (t2=2.2 days, t3=4.3 days) as has been found by us between +22 and +26 days (t2=30 days, t3=36 days). This fact suggests that the underlying physical mechanisms are similar.

The general appearance is typical for "He/N'' spectra defined by Williams (1992). This gives further support to the statement of Williams (1992), that almost all novae with short recurrence times appear to have "He/N'' type spectra (U Sco, V394 CrA, V745 Sco, V3890 Sgr, LMC 1990 No. 2). The interpretation of this classification was given by Williams (1992) in terms of different components in the ejecta. The broader lines of the "He/N'' spectrum originate in a discrete shell, which is ejected at considerably high velocities from the white dwarf surface at the peak of outburst.

In our dataset there are some weak pieces of evidence for another interpretation. The fairly strong and broad Fe II 5169 line around the maximum raises the possibility that CI Aql belongs to the hybrid objects as discussed in Williams (1992). These stars have "Fe II'' type spectra in the early phases that evolve to the "He/N'' type. The very early ($\Delta t$ = -7 d) low-resolution spectrum taken by W. Liller showed no evidence of Fe II lines (Yamaoka et al. 2000), while our spectrum a week later showed strong and broad Fe II 5169 emission (Fig. 1). Also, the intermediate resolution blue spectrum taken at +11 d (Fig. 2) is very similar to what is observed in Nova LMC 1988 No. 2 by Sekiguchi et al. (1989b). Williams (1992) suggested that this behaviour is caused by two distinct phases of the outburst. The "Fe II'' spectra probably originate from discrete and massive shells, which are optically thick, thus causing the early photosphere to occur in the ejected shell. As the expanding shell(s) becomes optically thin, the spectrum change to the "He/N'' type with more rectangular line shapes. The double-structured absorption in the P-Cyg profiles (Figs. 1 and  3) implies a quite complex inner structure of the ejecta, thus may strengthen the previous explanation.

The H$\alpha $ evolution is also similar to what has been found in other RNe. Munari et al. (1999) presented a nice coverage between +0.64 d and +22.6 d for U Sco, in which the early saddle-like H$\alpha $ split into three components with velocity separation of the order of $\pm $1600 km s-1. While the three components in the eclipsing system of U Sco are difficult to interpret in terms of collimated beams of material ejected at a large angle from the orbital plane, the two components of CI Aql can be explained with simple equatorial and polar rings of enhanced brightness in the ejected shell (Gill & O'Brien 1999). Since CI Aql is an eclipsing system, we see the hypothetic rings nearly edge-on. As has been pointed out by Gill & O'Brien (1999), the high inclination means that the least information on the shell structure can be derived from the line profiles.

Finally, the main conclusions of this paper can be summarized as follows:

Acknowledgements
This research was supported by the "Bolyai János'' Research Scholarship of LLK from the Hungarian Academy of Sciences, Hungarian OTKA Grant #T032258 and Szeged Observatory Foundation. The warm hospitality of the staff of the Konkoly Observatory and their provision of telescope time is gratefully acknowledged. The NASA ADS Abstract Service was used to access data and references. We also acknowledge the data service of the VSNET group. This research has made use of Simbad Database operated at CDS-Strasbourg, France.


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