THE CEPHEID PERIOD-LUMINOSITY ZERO-POINT FROM HIPPARCOS TRIGONOMETRICAL PARALLAXES

Hipparcos trigonometrical parallaxes of Cepheid variables are used to derive a zero-point for the period-luminosity (PL) relation. Adopting a slope from the Large Magellanic Cloud (LMC), the relation is found t o be [M(V)] = -2.81 logP - 1.43. The standard error of the zero-point is 0.10 mag. Together with metallicity corrections this corresponds to a distance modulus of 18.70 +/- 0.10 for the LMC and 24.77 +/- 0.11 f or M31. Some implications of these results are discussed. Estimates of the Hubble constant (H-0) that are based on Cepheid observations toge ther with an adopted LMC distance modulus of 18.50 will on average now need to be decreased by similar to 10 per cent. However, metallicity corrections, which have frequently been ignored, will result in the ac tual percentage change varying with the sample of galaxies studied. Ca libration of RR Lyrae absolute magnitudes using the LMC and M31 Cephei d distances implies an age for the oldest Galactic globular clusters o f similar to 11 Gyr. The parallax data show that the period of Polaris corresponds to first overtone pulsation.