Radial velocities & photometry of classical Cepheids

The distances are determined from BVI photometry, with semi-empirical metallicity corrections calibrated on the Magellanic Clouds. Using these Cepheids as tracers, the rotation curve of the disc is determined between R_0_ and 2R_0_.

The result is a flat rotation curve about 30km/s lower than theta_0_, V_rot_=193+/-4km/s for R_0_==8.5kpc and theta_0_=220kms/s assumed, or V_rot_=167+/-4km/s for R_0_=8kpc and theta_0_=200km/s. The possible presence of non-axisymmetric components in the rotation of the outer disc is considered. We find a very small or vanishing value for any radial motion of the LSR or expansion/contraction motion.