2. Observations and data reductions
The TNT telescope is equipped with a Tektronics CCD 512x512 pixels, with a total field of view of 4 x . Observational data were secured mainly during the 1995 campaigns: AQ Lyr from July 22 to 27; CN Lyr from July 25 to 27 (plus June 10, 1996); AW Dra from July 19 to 25. Single exposure times ranged from 30-90 sec in V to 120-300 in B filter, with a typical seeing of about . Several Bias and flat-field frames were taken at the beginning and at the end of each night; following the usual procedures, these frames were used to pre-reduce observational data.
In order to minimize the observation time and, consequently, to increase the sampling of the lightcurves, the magnitude of each variable has been obtained by evaluating the difference in magnitude between the target star and suitable comparison stars in the field of view of the telescope. Final data in standard B,V magnitudes have been obtained calibrating the comparison stars with several Landolt (1992) stars on three nights with good photometric conditions. Note that in this way one is also speeding up the reduction procedure, since no differential atmospheric extinction has to be taken into account in the magnitude differences between target and comparison stars.
The magnitudes of stars in each frame have been obtained through a suitable procedure for the ESO-MIDAS reduction package. Fig. 1 gives the identification maps for the three variables and for the chosen comparison stars. As shown in the figure, in all fields we chose two comparison stars. However, the comparison star C2 of AW Dra is suspected to be a variable star (see Fig. 2), whereas the non-variability of star C1 is ensured by the constancy of the AW Dra minima; so only the comparison star C1 was used.
In the other two cases, we derived a lightcurve with respect to each of the two comparison stars, thus averaging the two lightcurves to produce the final result. We found that differences between the two averaged curves are of the order of 0.01-0.02 magnitude. The magnitudes of the comparison stars in Fig. 1 are given in Table 1.
© European Southern Observatory (ESO) 1998
Online publication: April 28, 1998