2 Observations

The observations were obtained with the Dutch 0.9m telescope at La Silla Observatory (ESO) during ten consecutive nights (Oct. 13-22, 1996) by means of a TEK512 CCD with 580 columns and 520 rows (ESO chip #33). Each frame covers a field of view of 3.8$^\prime$ square. The characteristics of the instrumentation are described by Schwartz et al. (1995). A total of 28, 29 and 30 V frames were obtained for HV 1777, HV 1779 and HV 1353, respectively, with a typical exposure time of 5 minutes. During the last night some R frames were obtained besides the V and R frames at different airmasses of the two standard CCD fields Rubin 149A and SA 98-650 (Landolt 1988) in order to derive standard V and R magnitudes. The frames were reduced in the usual way by means of IRAF packages and using sky flat fields obtained both at sunset and dawn. Measurements were then performed by means of the aperture photometry package APPHOT.

From the observed colours of the 13 standard stars in the two fields we obtained transformation equations which allow to fit the standard colours with rms scatters of 0.009 and 0.007 mag in V and R respectively. Since our aim was to perform differential photometry between our Cepheids and some suitable comparison stars, and to detect other possible unknown Cepheids in the fields, all the brightest objects were measured, that is almost all the stars with . A total of 19, 9 and 21 stars in the fields of HV 1777, HV 1779 and HV 1353, respectively, were measured. The identification maps are reported in Figs. 1, 2, 3; each side is 3.8$^\prime$.

  

Figure 1: Field of HV 1353. Circles indicate the measured stars. "Var" is the Cepheid while "C" is the star adopted for computing differential magnitudes. North is up and Right Ascension increases towards right

  

Figure 2: Field of HV 1777. Symbols as in previous figure. "Var b" is HV 1763

  

Figure 3: Field of HV 1779. Symbols as in Fig. 1

The three panels of Fig. 4 show for each field the standard deviations of the differential magnitudes with respect to the brightest star versus the standard V magnitude. This figure allows to evaluate the intrinsic accuracy of the measurements. Apart from the three Cepheids there is also a strongly deviating object in the field of HV 1777. It corresponds to the variable HV 1763, detected by Leavitt (1908), but with unknown period.

  

Figure 4: Standard deviations of the magnitude differences between measured objects and the brightest one for each of the three investigated fields

Table 1 contains times and V magnitudes of the 4 variable stars.

  

Table 1: V photometric observations