We have selected the Seyfert galaxies by the following criteria: to be brighter than $16^{\rm m}$ in V-band and to have declination $>-10\hbox{$^\circ$ }$ . All the objects (listed in Table 1) are classified generally as Seyfert 1 type (Véron-Cetty & Véron 1998).

Observations were performed with the 0.6-m reflector of the Belogradchik Observatory, Bulgaria (Antov & Konstantinova-Antova 1985). Red-sensitive camera SBIG ST-8 and Johnson-Cousins set of filters (Fig. 1) were used. The chip of the camera is KAF 1600 (16 bit), with dimensions $13.8\times9.2$ mm or $1530\times1020$ pixels. The size of the pixel is $9\times9$ $\mu$ m. The scale is $27.5\hbox{$^{\prime\prime}$ }$ /mm or $0.25\hbox{$^{\prime\prime}$ }$ /pixel. The readout noise is 10 ADU/pixel and the gain 2.3 e^-/ADU. The camera allows binning $2\times2$ and $3\times3$ . In these cases the chip is $765\times510$ pixels and $510\times340$ pixels with scale $0.50\hbox{$^{\prime\prime}$ }$ /pixel and $0.75\hbox{$^{\prime\prime}$ }$ /pixel respectively. In all cases the covered field is $\approx$ $6\hbox{$^\prime$ }\times4\hbox{$^\prime$ }$ . The camera and the equipment are described in details in Bachev et al. (1999).

During the period of two years (1998-2000) more than 700 CCD frames (including standards used for calibration) have been obtained and analysed. The typical exposure time of these frames was 120 s. We used binning depending on the seeing. Binning $2\times2$ was used when seeing was $2\hbox{$^{\prime\prime}$ }$ or better and $3\times3$ was used when seeing was between $2\hbox{$^{\prime\prime}$ }$ and $3\hbox{$^{\prime\prime}$ }$ .

The magnitudes of the sequence stars have been calibrated using standard stars in the fields of stellar clusters - M 67 (Chevalier & Ilovaisky 1991) and in some cases NGC 7790, and M 92 (Christian et al. 1985). These fields are more convenient for use than the Landolt equatorial standards, because of the large number of stars that can be measured in a single frame. This is true especially for the high latitude observatories for which the air mass of the equatorial standards is significant even in culmination which might increase the errors of the extinction coefficients in reduction equations, and respectively - decrease the overall photometric accuracy. The accuracy of the standard stars photometry is indicated to be about $0\hbox{$.\!\!^{\rm m}$ }02$ (see for instance a comparison among different measurements by Chevalier & Ilovaisky 1991). For the calibration we preferred to use mostly brighter standards ( $10^{\rm m} -14^{\rm m}$ ) with no companions closer than $10\hbox{$^{\prime\prime}$ }$ .

All frames were dark subtracted and flat fielded with the standard camera's software. Photometry was performed with a code developed at the Observatory (Bachev et al. 1999). A comparison with DAOPHOT photometry routines for some of the measured stars showed no significant difference (not more than $\pm 0\hbox{$.\!\!^{\rm m}$ }01$ ) in the instrumental magnitudes. Aperture of $12\hbox{$^{\prime\prime}$ }$ was used for the measurements of all stars. The observations were made in clear nights with seeing typically $2\hbox{$^{\prime\prime}$ }-3\hbox{$^{\prime\prime}$ }$ . For each night the standard fields were observed at least two times and a linear reduction equation was obtained for each spectral band. CCD frames with calibrated stars and standards were taken preferably with air mass and time differences as small as possible in order to diminish the influence of the atmosphere.

Table 2: VRI magnitudes and errors of photometry (see text) of comparison stars in the fields of selected active galaxies. The stars and the AGNs are indicated in Fig. 2. The number of observational epochs is given for each field ( $N_{\rm Obs}$ )
Object $N_{\rm Obs}$ Star V ( $\sigma_{V}$ ) R ( $\sigma_{R}$ ) I ( $\sigma_{I}$ )

Mkn 335 7 A 10.96 (0.03) 10.65 (0.03) 10.36 (0.06)

B 14.25 (0.04) 13.65 (0.03) 13.11 (0.03)

C 15.05 (0.05) 14.76 (0.04) 14.50 (0.04)

D 15.40 (0.05) 15.00 (0.04) 14.61 (0.05)

Mkn 352 3 A 11.84 (0.03) 11.43 (0.03) 11.02 (0.06)

B 14.71 (0.04) 14.29 (0.04) 13.87 (0.06)

C 14.94 (0.03) 14.55 (0.03) 14.15 (0.06)

D 15.00 (0.04) 14.63 (0.04) 14.24 (0.06)

E 15.15 (0.03) 14.76 (0.03) 14.35 (0.06)

F 15.40 (0.04) 15.04 (0.04) 14.66 (0.06)

II Zw 1 3 A 12.71 (0.05) - -

B 12.94 (0.04) 12.60 (0.03) 12.25 (0.04)

C 13.81 (0.04) 13.50 (0.03) 13.18 (0.04)

D 14.89 (0.05) 14.49 (0.04) 14.08 (0.05)

E 15.24 (0.05) 14.83 (0.04) 14.41 (0.05)

Mkn 590 3 A 11.63 (0.03) 11.39 (0.03) 11.13 (0.05)

B 14.43 (0.04) 14.03 (0.05) 13.68 (0.06)

C 14.75 (0.06) 14.39 (0.05) -

D 15.52 (0.06) 15.22 (0.05) 14.96 (0.06)

Mkn 595 3 A 13.40 (0.03) 12.94 (0.03) 12.50 (0.04)

B 13.70 (0.03) 13.31 (0.04) 12.93 (0.05)

C 15.30 (0.03) 14.89 (0.04) 14.50 (0.05)

Mkn 618 2 A 10.72 (0.05) 10.17 (0.04) 9.60 (0.06)

B 11.89 (0.05) 11.37 (0.04) 10.85 (0.06)

C 13.52 (0.05) 13.07 (0.04) 12.61 (0.06)

D 14.49 (0.05) 14.26 (0.04) 13.87 (0.06)

E 15.05 (0.05) 14.55 (0.04) 14.10 (0.06)

Akn 120 4 A 11.02 (0.03) 10.50 (0.04) 10.00 (0.06)

B 12.35 (0.03) 11.81 (0.04) 11.26 (0.06)

C 13.93 (0.04) 13.42 (0.04) 12.95 (0.06)

D 14.67 (0.05) 14.37 (0.05) 14.10 (0.06)

Mkn 376 3 A 12.99 (0.03) 12.59 (0.03) 12.21 (0.04)

B 13.65 (0.04) 13.16 (0.04) 12.73 (0.05)

C 14.03 (0.04) 13.63 (0.04) 13.28 (0.05)

D 15.82 (0.06) 15.45 (0.05) 15.05 (0.05)

Mkn 9 3 A 12.09 (0.03) 11.69 (0.03) 11.32 (0.05)

B 12.76 (0.04) 12.23 (0.03) 11.74 (0.05)

C 13.37 (0.03) 12.94 (0.03) 12.56 (0.05)

D 15.27 (0.04) 14.89 (0.03) 14.55 (0.05)

E 15.94 (0.04) 15.49 (0.03) 15.10 (0.05)

Mkn 382 2 A 10.09 (0.03) - -

B 13.41 (0.03) 12.82 (0.03) 12.29 (0.04)

C 13.48 (0.03) 13.00 (0.03) 12.59 (0.03)

D 14.62 (0.03) 14.24 (0.03) 13.89 (0.03)

Mkn 279 7 A 12.11 (0.03) 11.55 (0.03) 11.04 (0.04)

B 12.96 (0.03) 12.53 (0.03) 12.16 (0.04)

C 14.89 (0.04) 14.32 (0.03) 13.79 (0.04)

D 15.59 (0.04) 15.15 (0.03) 14.76 (0.04)

Mkn 315 6 A 11.00 (0.04) 10.59 (0.03) 10.18 (0.05)

B 15.93 (0.05) 15.49 (0.03) 15.05 (0.05)

**Table 2:** *VRI* magnitudes and errors of photometry (see text) of comparison stars in the fields of selected active galaxies. The stars and the AGNs are indicated in Fig. 2. The number of observational epochs is given for each field ( $N_{\rm Obs}$ )
Object	$N_{\rm Obs}$	Star	V ( $\sigma_{V}$ )	R ( $\sigma_{R}$ )	I ( $\sigma_{I}$ )
Mkn 335	7	A	10.96 (0.03)	10.65 (0.03)	10.36 (0.06)
		B	14.25 (0.04)	13.65 (0.03)	13.11 (0.03)
		C	15.05 (0.05)	14.76 (0.04)	14.50 (0.04)
		D	15.40 (0.05)	15.00 (0.04)	14.61 (0.05)
Mkn 352	3	A	11.84 (0.03)	11.43 (0.03)	11.02 (0.06)
		B	14.71 (0.04)	14.29 (0.04)	13.87 (0.06)
		C	14.94 (0.03)	14.55 (0.03)	14.15 (0.06)
		D	15.00 (0.04)	14.63 (0.04)	14.24 (0.06)
		E	15.15 (0.03)	14.76 (0.03)	14.35 (0.06)
		F	15.40 (0.04)	15.04 (0.04)	14.66 (0.06)
II Zw 1	3	A	12.71 (0.05)	-	-
		B	12.94 (0.04)	12.60 (0.03)	12.25 (0.04)
		C	13.81 (0.04)	13.50 (0.03)	13.18 (0.04)
		D	14.89 (0.05)	14.49 (0.04)	14.08 (0.05)
		E	15.24 (0.05)	14.83 (0.04)	14.41 (0.05)
Mkn 590	3	A	11.63 (0.03)	11.39 (0.03)	11.13 (0.05)
		B	14.43 (0.04)	14.03 (0.05)	13.68 (0.06)
		C	14.75 (0.06)	14.39 (0.05)	-
		D	15.52 (0.06)	15.22 (0.05)	14.96 (0.06)
Mkn 595	3	A	13.40 (0.03)	12.94 (0.03)	12.50 (0.04)
		B	13.70 (0.03)	13.31 (0.04)	12.93 (0.05)
		C	15.30 (0.03)	14.89 (0.04)	14.50 (0.05)
Mkn 618	2	A	10.72 (0.05)	10.17 (0.04)	9.60 (0.06)
		B	11.89 (0.05)	11.37 (0.04)	10.85 (0.06)
		C	13.52 (0.05)	13.07 (0.04)	12.61 (0.06)
		D	14.49 (0.05)	14.26 (0.04)	13.87 (0.06)
		E	15.05 (0.05)	14.55 (0.04)	14.10 (0.06)
Akn 120	4	A	11.02 (0.03)	10.50 (0.04)	10.00 (0.06)
		B	12.35 (0.03)	11.81 (0.04)	11.26 (0.06)
		C	13.93 (0.04)	13.42 (0.04)	12.95 (0.06)
		D	14.67 (0.05)	14.37 (0.05)	14.10 (0.06)
Mkn 376	3	A	12.99 (0.03)	12.59 (0.03)	12.21 (0.04)
		B	13.65 (0.04)	13.16 (0.04)	12.73 (0.05)
		C	14.03 (0.04)	13.63 (0.04)	13.28 (0.05)
		D	15.82 (0.06)	15.45 (0.05)	15.05 (0.05)
Mkn 9	3	A	12.09 (0.03)	11.69 (0.03)	11.32 (0.05)
		B	12.76 (0.04)	12.23 (0.03)	11.74 (0.05)
		C	13.37 (0.03)	12.94 (0.03)	12.56 (0.05)
		D	15.27 (0.04)	14.89 (0.03)	14.55 (0.05)
		E	15.94 (0.04)	15.49 (0.03)	15.10 (0.05)
Mkn 382	2	A	10.09 (0.03)	-	-
		B	13.41 (0.03)	12.82 (0.03)	12.29 (0.04)
		C	13.48 (0.03)	13.00 (0.03)	12.59 (0.03)
		D	14.62 (0.03)	14.24 (0.03)	13.89 (0.03)
Mkn 279	7	A	12.11 (0.03)	11.55 (0.03)	11.04 (0.04)
		B	12.96 (0.03)	12.53 (0.03)	12.16 (0.04)
		C	14.89 (0.04)	14.32 (0.03)	13.79 (0.04)
		D	15.59 (0.04)	15.15 (0.03)	14.76 (0.04)
Mkn 315	6	A	11.00 (0.04)	10.59 (0.03)	10.18 (0.05)
		B	15.93 (0.05)	15.49 (0.03)	15.05 (0.05)

Table 3: Comparison between different calibrations
Work Miller 1981 Hamuy & Maza 1989 This paper

Object Star V V R I V R I

Akn 120
A - 11.07 10.53 10.06 11.02 10.50 10.00

B 12.42 12.38 11.82 11.29 12.35 11.81 11.26

C 13.96 13.91 13.39 12.91 13.93 13.42 12.95

D 14.68 14.68 14.34 14.01 14.67 14.37 14.10

Mkn 315 A 10.97 - - - 11.00 - -

B 15.95 - - - 15.93 - -

Work		Miller 1981	Hamuy & Maza 1989			This paper
Object	Star	V	V	R	I		V	R	I
Akn 120	A	-	11.07	10.53	10.06		11.02	10.50	10.00
	B	12.42	12.38	11.82	11.29		12.35	11.81	11.26
	C	13.96	13.91	13.39	12.91		13.93	13.42	12.95
	D	14.68	14.68	14.34	14.01		14.67	14.37	14.10
Mkn 315	A	10.97	-	-	-		11.00	-	-
	B	15.95	-	-	-		15.93	-	-

2 Observations and reductions