It is pointed out that observational work on Cepheids in the R, I photometric system offers a number of advantages: the stars are generally brighter at these wavelengths and so can be more easily and accurately observed; effects of binary companions, which are usually earlier type stars, are minimized; reddening is less, can be more easily determined, and is less liable to differing extinction laws than in the UBV system; line-blanketing problems are smaller; the range in (R — I)0 is probably less than in (B — V)0, and the importance of the colour term in the P-L-C relation thus reduced.
Available R, I data for classical Cepheids are gathered from the literature, and the P-L, P-C, and P-L-C relations investigated. It is found that the preliminary relations
represent the data to within ± 0.13 mag. in 〈M
〉 and ± 0.02 mag. in 〈R–I〉o.
There is some indication that the intrinsic colours of EV Set, U Sgr, and S Nor are bluer than hitherto believed, conceivably due to circumstellar reddening.