The Zero-Point of the Period-Luminosity-Relation of Cepheids

The zero-point of the period-luminosity-relation has been redetermined by applying the method of secular parallaxes. The determination presented here is based on proper motions, radial velocities and photometric data of about a hundred Cepheids of Population I. This approach was encouraged by the availabfflty of improved proper motions and photometric data. All proper motions were reduced to the system of FK4, and, for purposes of comparison, to the N30 system. The effects of galactic rotation, of incorrect precession, and of a motion of the equinox were taken into account. The radial velocities were discussed with respect to solar motion, differential galactic rotation and K-term. Adopting the slope of Kraft's period-luminosity-relation, the zero-point is found to be - 1' 88 + O 45 (m.e.). Kraft's value was -1' 67. With the slope of Fernie's period-luminosity-relation the zero-point is found to be 2 .05 + 0rn.45 (m.e.). Fernie obtained the value - 1rn.99.