3 The photometric sequences

Between 10 and 15 stars around each symbiotic star have been selected to form the comparison sequences, given in Table 2. The sequences have been selected and ordered on the basis of the B magnitude. The B magnitude is reproducible by most filter-equipped CCD cameras, it is the closest one to the $m_{\rm pg}$ band of the historical photographic observations and the Bband is particularly well suited to investigate the variability of symbiotic stars (see next section).

The range in magnitude of the sequences is large enough to cover both outburst and quiescence phases (eclipses included) of each symbiotic star. The comparison sequences are tighter around the usual brightness of the symbiotic stars and become looser away from it. In most cases the sequences extend to much fainter magnitudes (down to $B \geq 19$ mag or $B \geq 20$mag) than reached by the respective symbiotic stars because they could be of interest to other observational projects as well as in assisting in the calibration of sky survey projects on photographic plates.

For 9 objects (Draco C-1, ALS 2, K 3-9, V919 Sgr, Ap 3-1, V335 Vul, Hen 3-468, V627 Cas and StH$\alpha$ 32) the symbiotic star and the comparison sequence both lie inside a 5.16 $\times$ 5.16 arcmin field (see Fig. 1), which match in dimension the Allen (1984) finding charts. For the remaining 11 program objects, comparison stars bright enough to cover the outburst phases had to be found at greater distance from the symbiotic star. They are given in Table 2 at the end of each sequence, separated by an empty line from the other comparison stars, and are plotted on the less deep and wider finding charts of Fig. 3.

The stars included in the comparison sequences have been checked on at least three different nights for variability (see Col. N of Table 2). We cannot rule out beyond doubt that some of them are indeed variable (they could be eclipsing systems observed outside eclipse, for example), but the fairly good agreement (at a few millimag level) of their magnitudes as measured on different nights over some months gives some confidence in their use. Finally, to avoid problems of blending with nearby stars on plates or CCD images from short focus telescopes, the comparison stars have been selected so as to avoid those with close companions.