5 Summary and outlook

As a first test for our new filter set we observed a field in M 31. Plotting colour-colour diagrams of the photometry of the two broad band filters (V and i) and the two narrow band filters (TiO and CN), one can easily separate O- and C-rich AGB stars by photometric means. Besides we showed, that a special "CMD'' with i₀ and (TiO-CN)₀ can be used to sort out candidates, even without photometry in the V band. We found 286 (+221) new M- and 47 (+14) new C-star candidates in M 31. We enlargened the sample of known M 31 C-stars by 25%. Our derived properties of the AGB population in the studied M 31-field are consistent with those of Brewer et al. (1995).

A comparison between the results (e.g. star numbers) of all galaxies, which have been investigated so far with this technique by different groups (e.g. Brewer et al. 1995 or Albert et al. 2000), seems interesting. Unfortunately, this is not a "straightforward'' task, due to differencies in some details of their work (properties of the narrow band filters, selection criteria, ...).

Our future work will concentrate on other galaxies within the Local Group, where we want to study the AGB population. Observations have already been carried out.

Our observations will also form the basis of follow-up monitoring and spectroscopy. We want to enlarge the often very small samples of known extragalactic AGB stars to statistically meaningful numbers, derive LFs for M- and C-stars, compare the LFs among the different galaxies including the (better known) LFs of the LMC and SMC, and correlate them with their respective star formation histories.

Acknowledgements

This work was supported by APART (Austrian Programme for Advanced Research and Technology) from the Austrian Academy of Sciences, the Österreichische Nationalbank under the Jubiläumsfonds-project number 6876 and the Fonds zur Förderung der wissenschaftlichen Forschung under project number S7308-AST. The authors are very grateful to Rita Loidl, who provided us with synthetic spectra of C-stars. We used the Simbad database operated at CDS, Strasbourg, France.