Although the optical continuum variability is a well known feature of a significant part of Seyfert galaxies (Dibai & Lyutyi 1984; Hamuy & Maza 1987) it is still little known about the nature and characteristics of this variability. It is important to monitor the non-blazar type AGNs (Seyfert galaxies and QSOs) because the reasons for their variability are most probably connected with instabilities in an accretion disk around a supermassive black hole, unlike the case of blazars where similar changes are usually attributed to processes in a relativistic jet (Kawaguchi & Mineshige 1998; Ulrich et al. 1997). A detailed study of Seyferts' variability can, therefore, shed some light on the accretion disk structure and properties, and respectively - on the entire mechanism of gas supply to the central engine. Useful information could be obtained only in case of systematic campaigns, revealing well the temporal behaviour of AGNs on time scales from several hours to several years. This can be realised only if efforts of different observers and observatories are involved. Here the contribution of smaller observatories, equipped with 0.5-1-m telescopes and CCD cameras, could be very important. The use of different reference stars, however, could lead to systematic shifts in the measured brightness of the variable object from one observer to another. This is especially true if broadband filters with different transparencies and cameras with different sensitivities are used.

According to our knowledge, only for a small part of Seyfert galaxies convenient comparison star fields have been calibrated so far (Lyutyj 1971; Miller 1981; Hamuy & Maza 1989). Furthermore, some of these stars are not very suitable for a differential CCD photometry either because they are calibrated only in UBV bands or because of their significant distance from the galaxy (up to $30\hbox{$^\prime$ }$ or more), often exceeding the usually smaller CCD field ( $5\hbox{$^\prime$ }-15\hbox{$^\prime$ }$ ). Comparing different calibrations, one can conclude that the photometric accuracy usually reached is not better than 0 $.\!\!^{\rm m}$ 03. More measurements made by different researches could increase that accuracy or reveal a possible long-term variability of some of the stars.

That is why we, inspired by Petrov (1988), decided to start a work for calibration of close comparison stars in V (Johnson) and RI (Cousins) bands (where most of today used CCD cameras reach their maximal sensitivity) in the fields of a large number of northern, bright (in general $V<16^{\rm m}$ ) Seyfert galaxies and QSOs, selected according to Véron-Cetty & Véron (1998) catalogue. In this paper we present our first results - comparison sequences in the fields of 12 Seyfert galaxies (Table 1).

Table 1: The list of selected AGNs. Positions, other names, Seyfert types, and V-band magnitudes ( $V_{{\rm VCV98}}$ and $V_{{\rm This\, Work}}$ ), both measured in 16 $\hbox {$^{\prime \prime }$ }$ diaphragm. Co-ordinates, Seyfert types and $V_{{\rm VCV98}}$ are taken from Véron-Cetty & Véron (1998). $V_{{\rm This\, Work}}$ is the averaged V-band magnitude, measured by us (Sect. 3)
Object RA (2000.0) Dec (2000.0) Other Names Sy type V $_{{\rm VCV98}}$ V $_{{\rm This\, Work}}$

Mkn 335 00 06 19.5 +20 12 09 1.2 13.85 14.0

Mkn 352 00 59 53.3 +31 49 36 1.0 14.81 15.0

II Zw1 01 21 59.8 -01 02 24 Mkn 1503, MCG 0-40-98 1.5 15.17 15.3

Mkn 590 02 14 33.6 -00 46 00 NGC 863, UGC 1727 1.0 13.81 14.0

Mkn 595 02 41 34.9 +07 11 14 1.5 14.69 14.8

Mkn 618 04 36 22.3 -10 22 38 MCG-2-12-45 1.0 14.51 14.7

Akn 120 05 16 11.4 -00 09 00 Mkn 1095, UGC 3271 1.0 13.92 13.9

Mkn 376 07 14 15.1 +45 41 56 1.5 14.62 15.2

Mkn 9 07 36 57.0 +58 46 17 1.5 14.37 14.7

Mkn 382 07 55 25.3 +39 11 10 MGC 7-17-1 1.0 15.50 15.5

Mkn 279 13 53 03.5 +69 18 29 UGC 8823 1.0 14.46 14.0

Mkn 315 23 04 02.7 +22 37 21 II Zw 187 1.5 14.78 14.7

**Table 1:** The list of selected AGNs. Positions, other names, Seyfert types, and V-band magnitudes ( $V_{{\rm VCV98}}$ and $V_{{\rm This\, Work}}$ ), both measured in 16 $\hbox {$^{\prime \prime }$ }$ diaphragm. Co-ordinates, Seyfert types and $V_{{\rm VCV98}}$ are taken from Véron-Cetty & Véron (1998). $V_{{\rm This\, Work}}$ is the averaged V-band magnitude, measured by us (Sect. 3)
Object	RA (2000.0)	Dec (2000.0)	Other Names	Sy type	V $_{{\rm VCV98}}$	V $_{{\rm This\, Work}}$
Mkn 335	00 06 19.5	+20 12 09		1.2	13.85	14.0
Mkn 352	00 59 53.3	+31 49 36		1.0	14.81	15.0
II Zw1	01 21 59.8	-01 02 24	Mkn 1503, MCG 0-40-98	1.5	15.17	15.3
Mkn 590	02 14 33.6	-00 46 00	NGC 863, UGC 1727	1.0	13.81	14.0
Mkn 595	02 41 34.9	+07 11 14		1.5	14.69	14.8
Mkn 618	04 36 22.3	-10 22 38	MCG-2-12-45	1.0	14.51	14.7
Akn 120	05 16 11.4	-00 09 00	Mkn 1095, UGC 3271	1.0	13.92	13.9
Mkn 376	07 14 15.1	+45 41 56		1.5	14.62	15.2
Mkn 9	07 36 57.0	+58 46 17		1.5	14.37	14.7
Mkn 382	07 55 25.3	+39 11 10	MGC 7-17-1	1.0	15.50	15.5
Mkn 279	13 53 03.5	+69 18 29	UGC 8823	1.0	14.46	14.0
Mkn 315	23 04 02.7	+22 37 21	II Zw 187	1.5	14.78	14.7

We think that the secondary standards presented here will increase (almost by $50\%$ ) the number of Seyferts with convenient comparison stars in the field. Our aim is to encourage and facilitate the CCD-monitoring of these interesting objects in order to obtain more important information about their variability.

The paper is organised as follows: in Sect. 2 observations and data reduction are presented. Section 3 contains the results and a comparison with other published data (where available). A summary is presented in Sect. 4.

1 Introduction