3 $\vec{BV(RI)}_{\sf C}$ photometry

3.1 Transformations

The BV observations of NGC663 were transformed using mainly the photoelectric observations of Hoag et al. (1961). The photographic observations of the same authors were used only in transformation of V, because their photographic B photometry is systematically biased (see Fig. 3). The transformation equations for B and V are the following:

$$B = b - \hbox{0.002(63)} \times (b-v) + \hbox{12.801(6)},$$ (1)

$$V = v + \hbox{0.070(29)} \times (v-i) + \hbox{12.198(8)},$$ (2)

where the numbers in parentheses denote the rms errors of the coefficients with the leading zeroes omitted. The lower-case letters are instrumental differential magnitudes calculated with respect to a set of relatively bright constant stars. Standard deviations of residuals from Eqs. (1) and (2) were 0.024 and 0.056 mag, respectively.

Since till now no $(RI)_{\rm C}$ photometry was published for NGC663, in order to transform our observations in these bands to the standard system, we observed NGC663 on three photometric nights in September and October 2000 together with another open cluster, NGC7790. We have chosen NGC7790 instead of the usually observed Landolt (1992) fields for two reasons: (i) this cluster is much closer to NGC663 than the Landolt (1992) standards, (ii)  $(RI)_{\rm C}$ standard photometry of NGC7790 has been recently published by Stetson (2000).

Using the photometry of NGC7790 provided by Stetson (2000), we derived the following transformation equations:

$$R_{\rm C} = r + \hbox{0.031(24)} \times (r-i) + \hbox{11.909(4)},$$ (3)

$$I_{\rm C} = i + \hbox{0.002(25)} \times (r-i) + \hbox{11.512(4)},$$ (4)

where the designations are similar to those in Eqs. (1) and (2), and the standard deviations of residuals from Eqs. (3) and (4) are equal to 0.011 and 0.012 mag, respectively. The resulting standard $BV(RI)_{\rm C}$ photometry of over 1200 stars is given in Table 2, available in the electronic form only, via anonymous ftp to CDS, Strasbourg (130.79.128.5). In Fig. 7, the $R_{\rm C}$vs.  $(R-I)_{\rm C}$ CM diagram is shown.

Figure 3: Comparison of BV photometries. All differences, $\Delta B$and $\Delta V$, are in the sense our minus the other. The other photometries are designated by the following letters in the left upper corners of panels: a) Hoag et al. (1961), photoelectric; b) Hoag et al. (1961), photographic; c) Phelps & Janes (1994); d) McCuskey & Houk (1964); e) Moffat & Vogt (1974), f) van den Bergh & de Roux (1978). The photoelectric data are shown with dots, photographic data, with open circles, and CCD data, with crosses.

The photometry we provide in Table 2 is not homogeneous in the sense that the central regions were observed more frequently than those situated at the borders of the P field. In consequence, stars of the same magnitude, but from different regions, may have photometry which differs in accuracy by a factor of up to 5. For this reason, the photometric errors are also given in Table 2.

3.2 Comparison with previous work

Having transformed our BV photometry to the standard system, we compared it with the following previous photometric studies: CCD measurements of Phelps & Janes (1994), photoelectric photometry of Hoag et al. (1961), photographic data of the same authors as well as those of McCuskey & Houk (1964), Moffat & Vogt (1974), and van den Bergh & de Roux (1978). Results of this comparison are shown in Fig. 3.

Photoelectric measurements of Hoag et al. (1961) were the primary source of standard stars used in our transformations. Hence the scatter in panel A of Fig. 3 is small. Out of four photographic photometries, the best agreement and the smallest scatter are shown by the photometry of van den Bergh & de Roux (1978). The differences between our photometry and the photographic photometries of Moffat & Vogt (1974) and McCuskey & Houk (1964) have larger scatter, while that of Hoag et al. (1961), especially the B photometry, exhibits a clear systematic, magnitude-dependent effect.

Surprisingly, the CCD photometry of Phelps & Janes (1994) also differs systematically from our measurements. While for stars with magnitudes in the range between 12 and 16 in B and V the agreement is quite good (apart from a 0.06 mag shift in B), for stars fainter than 16 mag, the photometry of Phelps & Janes (1994) is systematically fainter and shows a large scatter. In addition, stars brighter than $\sim$12 mag are also systematically fainter in their photometry. The same effect can be seen in both Band V, reaching 0.5 mag for stars of the 10th magnitude.