2. Optical observations
AG Dra is included in a long-term programme of optical photometry at the 60 cm Cassegrain telescope of Sonneberg Observatory. A computer controlled, photoelectric photometer with a diaphragm of , diameter is used to obtain consecutive UBV images of AG Dra, the comparison star BD+67 952 and a third star (SAO 16935) used for check purposes. Typical integration times were 20 sec and the brightnesses were reduced to the international system according to Johnson. The mean errors in the brightness determination are 0.01 mag in V and B and 0.03 mag in U. The photometric observations since June 1990 (for which near-simultaneous X-ray measurements with ROSAT are available) are tabulated in Table 1 and the U and B magnitudes are also plotted in Fig. 3. Earlier Sonneberg photometric data of AG Dra are included in Hric et al. (1993).
Table 1. Photometric observations of AG Dra at Sonneberg Observatory (see paragraph 2 for details).
In addition to the photoelectric data we have also used the photographic sky patrol of Sonneberg Observatory to fill up gaps in the photoelectric coverage. The sky patrol is performed simultaneously in two colors (red and blue). The brightness measurements of AG Dra from the blue plates covering the time interval of the two optical outbursts are given in Table 2 and are also plotted in the top panel of Fig. 3.
Table 2. Photographic observations of AG Dra at Sonneberg Observatory (see paragraph 2 for details).
© European Southern Observatory (ESO) 1997
Online publication: June 5, 1998