DELTA-ALPHA AND STROMGREN PHOTOMETRY OF STARS IN THE RENSON-CATALOG OF AP AND AM STARS

We have observed 131 stars of Renson's (1991) catalogue of Ap and Am s tars both in the Stromgren & Maitzen's (1976) Delta a system as a cont ribution to the photometric studies of the lambda 5200 broad band flux depression feature in chemically peculiar stars. With few exceptions the probability grouping of Renson for membership in the CP2 group of peculiar stars is nicely reflected by peculiar values of Delta a. Comp arison with already available Delta a values yields slightly larger va lues due to a minor shift in the filter g(1) sampling the depression. As found by Maitzen & Vogt (1983) the Geneva system peculiarity parame ters correlate well with Delta a. This study demonstrates the advantag eous performance of a photoelectric photometer with a rapidly rotating filter wheel moving in a stop and go mode.