An histogram of the number N of variable stars as a function of the
binned observed amplitude A, in steps of 0.5 magnitude, and for all
stars with definite amplitude is represented in Fig. 3 (click here). For
50% of the stars, the amplitudes range between 1.0 and 3.0, with a
maximum for 
. Ascribing magnitude 19 at brightness
minimum to the stars with 
, one can build the
corresponding histogram for almost all the catalogued stars
(Fig. 4 (click here)). This approximation is believed to be realistic for
long period variables (most of the present stars) since:
(i) the magnitude limit achieved on a 30 mn exposure R plate is about 19.5;
(ii) the thirteen R magnitudes determined for each star are well distributed over a complete cycle when phased together for typical periods of the detected variables (between 70 and 500 days).
The distributions displayed by Figs. 3 (click here) and 4 (click here) (in
the latter, 50% of the stars have amplitudes between 1.5 and 3.5),
are quite similar (the 0.5 magnitude shift observed between the two
histograms is not significant). The (ii) statement is supported by a
recent study of field O (Alard et al. 1996) showing
that 122 of the 150 variables selected for their large amplitude
variations (
) are Mira type stars, most of them
with periods between 150 and 500 days. The analogy shown by the
distributions N/A (Fig. 4 (click here)) and N/Period (Alard et al.
1996) corresponds
to the period-amplitude relation existing
for this class of stars. We plan to characterize Mira type stars
in other parts of the 
field of our
programme. These high luminosity objects show a definite
period-luminosity relation and thus are good distance indicators.
Infrared J and K photometry of these stars would then be
worthwhile in order to evaluate the variation of the interstellar
extinction with the distance modulus in the direction of the
galactic centre.
Acknowledgements
We would like to thank all the members of the MAMA team for assistance during the scanning of the photographic material. We are indepted to Gérard Marichy and Daniel Robert for their technical help.
