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, 
; B, V, R,
plates) by A.T. continued, after 1976, at the 1 m
Schmidt telescope (f/3, 
; U, B,
V, R, 
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 (
) (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 
, 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 
(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