3 Results

Finding charts with field $15\hbox {$^\prime $ }\times 15\hbox {$^\prime $ }$ for all observed sequence stars are presented in Fig. 2. The charts were retrieved from the STScI Digitized Sky Survey. North is at the top and east is to the left. On each chart the secondary standards and the Seyfert galaxy are indicated. All the sequence stars are closer than $6\hbox{$^\prime$ }$ (typically $3\hbox{$^\prime$ }-4\hbox{$^\prime$ }$) from the Seyfert galaxy. Their brightness is between $10^{\rm m}$ and $16^{\rm m}$. VRI magnitudes for all measured stars are listed in Table 2. The stars are labeled A, B, C... in order of their V-band magnitude. Errors of the photometry (a square root of the sum of squares of the dispersion of the total magnitude as a mean of different observational runs and the error of calibration, which is about $0\hbox{$.\!\!^{\rm m}$ }02-0\hbox{$.\!\!^{\rm m}$ }03$) are given. The magnitudes of stars around the active galaxy were measured in usually 3-4 photometric nights (Table 2). No variable stars (within $\pm0\hbox{$.\!\!^{\rm m}$ }03$) among the selected sequence stars have been detected by means of differential photometry with other stars in the field.

We found photoelectric calibrations (published after 1980) for several stars from our list (Miller 1981; Miller 1986; Hamuy & Maza 1989). A comparison with our measurements is presented in Table 3 and shows a good agreement between the magnitudes, assuming photometric errors of about $0\hbox{$.\!\!^{\rm m}$ }03-0\hbox{$.\!\!^{\rm m}$ }04$. It should be pointed out that the observers have used different photometry diaphragms in their measurements $-12\hbox{$^{\prime\prime}$ }$ - in this study, $17\hbox{$^{\prime\prime}$ }$ - by Hamuy & Maza (1989).

We measured V-band magnitudes of all Seyferts in a $16\hbox{$^{\prime\prime}$ }$ diaphragm (Table 1) in order to compare these magnitudes with those given by Véron-Cetty & Véron (1998) for the same diaphragm. The comparison shows clear indications for variability in some of the objects (Mkn 9, Mkn 279 and Mkn 376). It should be taken into account, however, that we subtracted the background using an annulus located just outside the photometry diaphragm while in photoelectric observations usually the background contribution is measured far from the object of interest. In our case this might slightly overestimate the measured magnitudes (for these objects - not more than $0\hbox{$.\!\!^{\rm m}$ }1$). In more details we explored the broadband variability of three objects - Mkn 279 (Bachev 2000), Mkn 315 and Mkn 335 (Bachev & Slavcheva-Mihova 2000). Monitoring of the other galaxies is continuing and the results will be published elsewhere.