Hubble Space Telescope Fine Guidance Sensor Parallaxes of Galactic   Cepheid Variable Stars: Period-Luminosity Relations
  (abridged) We present new absolute trigonometric parallaxes and relative proper motions for nine Galactic Cepheid variable stars: l Car, zeta Gem, beta Dor, W Sgr, X Sgr, Y Sgr, FF Aql, T Vul, and RT Aur. We obtain these results with astrometric data from Fine Guidance Sensor 1r, a white-light interferometer on Hubble Space Telescope. We find absolute parallaxes with an average sigma_pi/pi = 8%. Two stars (FF Aql and W Sgr) required the inclusion of binary astrometric perturbations, providing Cepheid mass estimates. With these parallaxes we compute absolute magnitudes in V, I, K, and Wesenheit W_{VI} bandpasses corrected for interstellar extinction and Lutz-Kelker-Hanson bias. Adding our previous absolute magnitude determination for delta Cep, we construct Period-Luminosity relations for ten Galactic Cepheids.   We compare our new Period-Luminosity relations with those adopted by several recent investigations, including the Freedman and Sandage H_0 projects. Adopting our Period-Luminosity relationship would tend to increase the Sandage H_$ value, but leave the Freedman H_0 unchanged. Comparing our Galactic Cepheid PLR with those derived from LMC Cepheids, we find the slopes for K and W_{VI} identical in the two galaxies within their respective errors. Our data lead to a W_{VI} distance modulus for the Large Magellanic Cloud, m-M = 18.50\pm0.03, uncorrected for any metallicity effects. Applying recently derived metalllcity corrections yields a corrected LMC distance modulus of (m-M)_0=18.40\pm0.05. Comparing our Period-Luminosity relationship to solar-metallicity Cepheids in NGC 4258 results in a distance modulus, 29.28 \pm 0.08, which agrees with that derived from maser studies. 
G. Fritz Benedict, Barbara E. McArthur, Michael W. Feast, Thomas G.   Barnes, Thomas E. Harrison, Richard J. Patterson, John...