Astron. Astrophys. 335, 545-548 (1998)

2. Observations and data analysis

The photoelectric observational material discussed in this paper was obtained in the framework of the international campaign for symbiotic stars launched by Hric and Skopal (1989). All data were published in a few papers of this series (see Hric et al. 1996 and references therein). For our analysis we used data including some from the time of activity of AG Dra in 1985 - 86 (the 2 relatively low light maxima are designated b1 and b2 by Viotti et al. 1996) and from the following epoch of quiescence. Light curves in the U, B and V colours are shown in Figs. 1 - 3.

Fig. 1. Photoelectric observations of AG Dra in the U colour.

Fig. 2. Photoelectric observations of AG Dra in the B colour.

Fig. 3. Photoelectric observations of AG Dra in the V colour.

We have in the following figures shown all data in phase diagrams using the ephemeris of Meinunger (1979):

Each cycle is marked by its own symbol to distinguish better the temporal development of the photometric behaviour. Observations in the first orbital cycle are denoted by crosses, those of the second by open circles, the third by diamonds, the fourth by open stars, the fifth by snow flakes and the sixth by dots. It should be noted that Meinunger's period (554 days) coresponds better to the minima and not very exactly to maxima. In a following step we superposed data in such a way that maxima of each cycle overlap each other. The phase diagrams in U, B and V colours are shown in Figs. 4 - 6, respectively, while overlapped cycles are displayed in Fig. 9 for U band.

Fig. 4. The phase diagram of the light curve of AG Dra according Meinunger's ephemeris in the U colour.

Fig. 5. The phase diagram of the light curve of AG Dra according Meinunger's ephemeris in the B colour.

Fig. 6. The phase diagram of the light curve of AG Dra according Meinunger's ephemeris in the V colour.

We see firstly that variations are especially pronounced in U (the amplitude of 1 mag). The amplitude of all variations (including outburst activity) decreases towards longer wavelengths (see Figs. 1 - 3). Fig. 4 shows that the minima in U appear to be relatively regular during the activity of 1985 - 86. The maxima which differ however to a large extent during activity, still differ somewhat during quiescence. The variations in B and V are small in quiescence, with the amplitudes of 0.3 and 0.2 mags respectively. The latter appear to be fairly well correlated with each other, but much less correlated with the variations in U. At some times one even may have the impression of an anticorrelation.

We have obtained from the period analysis by the phase dispersion minimization method by Stellingwerf (1978) and by Fourier transformation method (Kurtz 1985) new periods of 592 days (7 % longer than the Meinunger period) for U, 356 days for B and 350 days for V respectively for data obtained between outbursts (JD 46910 - 49098). In support of our new periods in B and V bands we show phase diagrams of the light curves with these periods in Figs. 7 and 8. The light variations are much more visible in these diagrams than in the previous ones. We suggest that this result indicate reality of the period around 350 days. The period analysis in the U colour involving only 4 maxima in quiescence may be affected by the postoutburst activity of this star as well as by the lack of exact periodicity found by us. Therefore we doubt the significance of the difference between our value for the period and the orbital one. We detect an absolutely new period around 350 days in the B and V bands, which we might connect with a mechanism in the vicinity of the cool component or in its atmosphere. What is particularly noticeable is that the shorter period is quite close to the interval between the 2 peaks observed by us in activity, which is also near 350 days in B and V.

Fig. 7. The phase diagram of the light curve of AG Dra with 356 days period in the B colour.

Fig. 8. The phase diagram of the light curve of AG Dra with 350 days period in the V colour.

Fig. 9. The overlap cycles of light variation of AG Dra in U are presented for better separation of each cycle from the others.

© European Southern Observatory (ESO) 1998

Online publication: June 18, 1998
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