2 Observations

The observations were taken with the 0.40 m Automatic Imaging Telescope at the Perugia Astronomical Observatory (Tosti et al. 1996). The telescope is mainly devoted to the monitoring of a large sample of blazars (Fiorucci & Tosti 1996b), however, a fraction of the telescope time is dedicated to the photometric observations of DNs. The telescope is equipped with a CCD camera and B, V (Johnson) and $R\rm _c$, $I\rm _c$ (Cousins) filters. Our photometric system has carefully been tested by observing both the M 67 sequence (Chevalier & Ilovaisky 1991) and Landolt stars (Landolt 1983a,b, 1992). No systematic errors were detected (see Fiorucci & Tosti 1996a).

All the CCD frames were corrected for bias and dark signal. Because of the high grade of uniformity of our CCD chip the usual flat field correction of the images was not required (see Tosti et al. 1996). The CCD frames were processed by REDUCE, the automatic reduction software described in Tosti et al. (1996). This photometry package, which is based on the algorithms described by Stetson (1987), finds and recognizes the stars in the frame and calculates the instrumental magnitudes through synthetic aperture photometry. The magnitudes reported in this paper have been obtained using an aperture radius of 4 arcsecs.

The data on AL Com were calibrated using Landolt stars because of the lack of sufficiently bright comparison stars within the limits of our CCD camera field of view. The correct finding chart for AL Com can be found in Howell et al. (1996).

For all the other dwarf novae reported in Table 1 we performed differential photometry using some non-variable stars present in their fields (see the finding charts shown in Figs. 1-4). The finding chart of V660 Her was already given by Downes & Shara (1993) but it was incorrect. In Fig. 4 we report the correct one.

In order to obtain B, V, $R\rm _c$, $I\rm _c$ secondary standard sequences in the field of the dwarf novae, the comparison stars were calibrated by observing, on photometric nights, several standard stars (Landolt 1983a,b, 1992) having (B-V) from -0.2 to 1.2, over a wide range of airmasses. The standard stars observations were then used to define the following transformations to the standard system:

$B-b_0 = \alpha_b + \beta_b (b-v)_0$

$V-v_0 = \alpha_v + \beta_v (v-r)_0$

$R_{\rm c}-r_0 = \alpha_r + \beta_r (v-r)_0$

$I_{\rm c}-i_0 = \alpha_i + \beta_i (v-i)_0$.

In the above equations, the atmospheric extinction corrected instrumental magnitudes are denoted by the index ("$\circ$''). The values of the $\alpha$ and $\beta$ coefficients were estimated via least-squares linear regressions on each photometric night. Table 2 lists only the mean transformation coefficients, their standard errors, and the typical standard deviations of the fit ($\rm \sigma_f$). As a result of the stability of our photometric system, we did not find appreciable differences among the values reported in Table 2 and the ones given in Fiorucci & Tosti (1996a).

  

Table 2: Transformation coefficients to the standard system

  

Figure 1: Finding chart of V544 Her. North is at the top and east is to the left. The box is 10 arcmin wide

  

Figure 2: Finding chart of V660 Her. North is at the top and east is to the left. The box is 10 arcmin wide

  

Figure 3: Finding chart of V516 Cyg. North is at the top and east is to the left. The box is 10 arcmin wide

  

Figure 4: Finding chart of DX And. North is at the top and east is to the left. The box is 10 arcmin wide

The standard magnitudes of the comparison stars reported in Table 3 are the weighted means of the values obtained during at least three photometric nights.

The B and $R\rm _c$ magnitudes of the comparison stars in the field of DX And are in good agreement with the measurements carried out by Drew et al. (1993).

Tacking into account of the typical errors in the measurement of the instrumental magnitudes (0.02 in V, $R\rm _c$, $I\rm _c$, and 0.03 in B), the contribution of the colour terms present in the above transformation equations results to be negligible, at least over the range of colour indices of the comparison and variable stars here considered. Therefore, the comparison stars were used to find the value of the zero point of the magnitude scale for each CCD image and, then, to measure the dwarf nova standard magnitude and its error (see Tables 4, 5, 6, 7 and 8).

  

Table 3: $BVR_{\rm c}I\rm _c$ magnitudes of the selected comparison stars

  

Table 4: $BVR_{\rm c}I\rm _c$ magnitudes of AL Com

  

Table 5: $BVR_{\rm c}I\rm _c$ magnitudes of V544 Her

  

Table 6: $BVR_{\rm c}I\rm _c$ magnitudes of V660 Her

  

Table 7: $BVR_{\rm c}I\rm _c$ magnitudes of V516 Cyg

  

Table 8: $BVR_{\rm c}I\rm _c$ magnitudes of DX And