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2. Observations and data reduction

Our programme is based on photographic observations made in 1968 (june and july) at the 48'' Schmidt telescope of Mount Palomar Observatory (f/2.44, tex2html_wrap_inline1412; B, V, R, tex2html_wrap_inline1420 plates) by A.T. continued, after 1976, at the 1 m Schmidt telescope (f/3, tex2html_wrap_inline1422; U, B, V, R, tex2html_wrap_inline1432 plates) of ESO (Chile) by local staff observers.

For the present survey thirteen R photographic plates have been retained for their photometric quality, at observational time intervals suitable for the detection of long-period variable stars. The characteristics of the plates (plate number, date of the observation, emulsion, filter, exposure, and seeing conditions) are given in Table 2 (click here). The comparison of photographic plates was done by one of us (A. T.) using the blink microscope of the Observatoire de Lyon, an instrument allowing stellar variations as small as 0.2 magnitude to be estimated on plates obtained in good seeing conditions (tex2html_wrap_inline1436) (Terzan et al. 1978). We found 1061 new variable stars in field C, in addition to the 130 variables already detected in the area common to fields O and C (Terzan et al. 1982). Magnitudes have been estimated by scaling the photographic densities of variables to a sequence of densities corresponding to standard stars with R magnitudes between 9 and 17.5 (Terzan & Bernard 1981). Under these conditions, the uncertainty on R is about 0.3 magnitude in the range tex2html_wrap_inline1442, and then increases up to 0.5 as R approaches 18, the magnitude limit of the estimations. The determination of the rectangular coordinates X and Y of the stars was made partly at ESO Garching with the microdensitometer S 3000- Optronics, partly with MAMA (Machine Automatique à Mesurer pour l'Astronomie). The astrometric reductions were performed using the PPM catalogue (Röser et al. 1991, 1993), with an internal accuracy of tex2html_wrap_inline1450 (see e.g. Robichon et al. 1996).

Table 1: Location of the investigated areas and number of detected variable stars

Table 2: Characteristics of the plates

Table 3: The first 16 new variable stars discovered in Field C

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