2 Observations

All observations of NGC663 were carried out at the Biaków Observatory of the Wrocaw University with a 60-cm telescope equipped with a $576 \times 384$ pixel CCD camera and an autoguider. Since the camera field of view is a 6$^\prime$ $\times$ 4$^\prime$rectangle, 15 overlapping fields were covered in order to observe most of the cluster stars. The borders of the observed field, called hereafter the "P field'', are shown with a solid line in Fig. 1, while a detailed map, showing all stars we measured, is given in Fig. 2.

Figure 1: A 30$^\prime$ $\times$ 30$^\prime$ fragment of the POSS I plate centred on NGC663. Solid lines show the border of the area covered by our $BV(RI)_{\rm C}$H$\alpha$ observations (P field). The dashed lines delimit the region searched for variability by Pigulski et al. (2001); we call it the V field. Three Be stars from outside the P field are encircled and labeled. North is up, east to the left.

Figure 2: Schematic view of our fifteen 6$^\prime$ $\times$ 4$^\prime$ fields in NGC663, covering the P field. The diameters of the circles correspond to the stars' images as seen through the $R_{\rm C}$ filter. All emission-line objects listed in Table 1 are labeled with their numbers from Sanduleak (smaller than 30) or Wallenquist (1929, larger than 30) list. North is approximately up, east to the left.

The observations in each field consisted of a series of frames taken through filters reproducing the $BV(RI)_{\rm C}$ bands of the Johnson-Cousins (Johnson & Morgan 1953; Cousins 1976) system and a single frame taken through the narrow H$\alpha$ filter. The latter was usually sandwiched between two frames taken through the wide H$\alpha$ filter. The observations were carried out during 10 nights between March 7, 1997 and September 1, 1998. Typical exposure times were the following: 2000 s for narrow H$\alpha$, 600 s for wide H$\alpha$ and B, and less than 200 s for the remaining filters.

NGC663 was also observed in 1999 in the above-mentioned program of searching for variable B-type stars in the northern open clusters. During these observations, however, only four slightly overlapping fields were observed in V and $I_{\rm C}$ bands. The area covered during this search is shown in Fig. 1 with dashed lines and will be called hereafter the "V field''. The results of the variability survey in the V field will be published separately (Pigulski et al. 2001). In this paper we present only that part of these results which refer to the variability of Be stars (see Sect. 6).

The CCD frames were calibrated in a standard way (see, e.g., Jerzykiewicz et al. 1997) and then reduced using the point-spread-function fitting in the Daophot II package of Stetson (1987).