Classification of intrinsic variables. VI. The long-period Cepheids.

A large number of UBV and RI observations of long-period very young disk Cepheids and halo Cepheids are examined. The sample includes l Carinae, b Carinae, and RS Puppis from the Galaxy, 10 SMC Cepheids, 11 LMC Cepheids, and giants and subgiants in M2, M10, M5 and Omega Centauri. From the data it emerges that the stellar associations to which it had been thought that 1 Car and RS Pup belonged do not exist. The reddening of Galactic Cepheids is less than usually supposed. The LMC and Galactic Cepheids may be very similar. The SMC long-period Cepheids probably have slightly lower metal abundance than LMC and Galactic Cepheids and almost certainly are hotter at a given period. Halo Cepheids with periods longer than about 30 days are of the same luminosity, M(V) = -3.2 mag, if M(V) = +0.6 mag for the very short period variables.