GALACTIC KINEMATICS OF CEPHEIDS FROM HIPPARCOS PROPER MOTIONS

The Hippnrcos proper motions of 220 Galactic Cepheids, together with r elevant ground-based photometry, have been analysed. The effects of Ga lactic rotation are very clearly seen. Mean values of the Oort constan ts, A = 14.82 +/- 0.84 km s(-1) kpc(-1) and B = -12.37 +/- 0.64 km s(- 1) kpc(-1), and of the angular velocity of circular rotation at the Su n, Omega(o) = 27.19 +/- 0.87 km s(-1) kpc(-1), are derived. A comparis on of the value of A with values derived from recent radial velocity s olutions confirms, within the errors, the zero-points of the period-lu minosity and period-luminosity-colour relations derived directly from the Hipparcos trigonometrical parallaxes of the same stars. The proper motion results suggest that the Galactic rotation curve is declining slowly at the solar distance from the Galactic Centre [(d Theta/dR)(o) = -2.4 +/- 1.2 km s(-1) kpc(-1)]. The component of the solar motion t owards the North Galactic Pole is found to be + 7.61 +/- 0.64 km s(-1) . Based on the increased distance scale deduced in the present paper, the distance to the Galactic Centre derived in a previous radial veloc ity study is increased to R-o = 8.5 +/- 0.5 kpc.