VRI photometry III: photographic and CCD R and I bands and the Kron-Cousins RI system

The Kron-Cousins R band has an extended red tail which, for M stars, causes large nonlinear differences between the standard R magnitudes and those measured using IIIa-F plates, or CCDs with rectangular passband filters. Accurate zero-point calibration ofthe nonstandard magnitudes can be made by using cubic transformations to convert the standard magnitudes ofthe calibrating stars into magnitudes that are linear in the natural system. Exact inverse transformations from the natural system will provide standard magnitudes and colors for stars of most spectral types. This discussion is supplemented by a note about B and V photometry with IHa-J and IIIa-F plates and CCD detectors. Key word: photometry