5 General characteristics of the sample variables

Semiregular variables clearly dominate our sample of late type giants. 583 light curves have been analysed in total. 112 of them have been classified as other. Among the remaining 471 objects we classified 18% as regular, 67% as semiregular and 15% as irregular. Due to different classification criteria a comparison of this result with other investigations is difficult. Cioni et al. (2001) found that 65% of the AGB stars are variable. As in our sample semiregular variables (in their case small amplitude red variables) are the dominant group of objects, only 12% of their stars have been classified as Miras.

If more than one period is detected, we have chosen a "primary'' period based on its amplitude. However, this decision is somewhat subjective when the amplitudes are similar. "Second periods'' are present in at least 54% of the stars classified as semiregular and about 18% of the regular variables. For the semiregular variables the ratio between the short and long period is between 1 and 2 for about 36% of the objects (Fig. 5), the other semiregulars have period ratios between 2 and 15. This result is similar to what has been found in previous investigations of SRVs in our Galaxy (e.g. Kiss & Szatmary 2000). Among the regular variables with two periods about one third of the stars show a period ratio below 2.

Figure 5: Ratio between the short and the long period of semiregular variables with two periods.

Comparing amplitude with period shows a maximum amplitude for periods between 300 and 400 days, given our 900-day window. No large amplitude variables with periods below 100 days have been found. Details are illustrated in Fig. 6 where the mean amplitude has been calculated for each period bin. Alard et al. (2001) found a similar increase of mean amplitude from small to long periods in a sample of AGB variables in the Galactic Bulge. However, their sample shows a maximum amplitude at periods around 250 days, as they excluded all miras from their analysis. Figure 7 summarizes the amplitude distributions within each class of variables. All three classes are dominated by small amplitude objects. Large amplitude stars occur exclusively among regular variables, all irregular objects are small amplitude stars.

Figure 6: Mean amplitude for each period bin. Only the primary period has been used in this plot.

We will now mainly concentrate on regular and semiregular variables. Figure 8 shows the period distribution of the AGAPEROS sample. Both groups of objects (regular and semiregular) show a maximum at the shortest periods: therefore, the regular variables cannot be simply related to the class of Mira variables. No Galactic Mira with a period below 100 days is known. Furthermore, Miras typically show large amplitude variability while the short period regular stars in our sample all have small amplitudes. The "classical'' Miras probably form the second maximum in period distribution of the regular variables around 350 days, similar to the Galactic Miras. We conclude that the group classically known as semiregular variables may contain a substantial number of periodic variables. On the other hand it is known from long time series of Galactic semiregular variables that these stars can show phases of periodic behavior, so that these stars may exhibit semiregular behavior on longer time scales.

Figure 7: $R_{\rm EROS}$ amplitude distribution for regular, semiregular and irregular variables, respectively. As discussed in Sect. 3, the amplitudes are estimated visually from the light curves.

Figure 8: Period distribution of regular and semiregular AGAPEROS variables. If a star has two periods, both have been included. Periods are given in days. The period determination is based on Fourier analysis as discussed in Sect. 3.