Abstract
Globular clusters are main astrophysical laboratories to test and modify evolutionary models. Thought to be rather homogeneous in their local elemental distribution of members, new results suggest a wide variety of chemical peculiarities. The preselection of apparent peculiar stars for a detailed spectroscopic analysis is very important for globular cluster fields. Most regions are very dense and the target stars are, normally, very faint. Photometry could be one way out of the dilemma since it is very efficient. Up to now, only observations in the Johnson UBV(RI) and Strömgren uvbyβ systems are able The tool of Δa photometry is employed in order to detect chemically peculiar Population II stars. This three filter narrow band system measures the flux distribution in the region from 4900 to 5600Å in order to find any peculiarities around 5200Å. The first Δ a observations for 3 globular clusters: NGC 104, NGC 6205 and NGC 7099, give very promising results, which will serve as a solid basis for follow-up observations including photometric as well as spectroscopic studies.
