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.
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.