The luminosities and distance scales of type II Cepheid and RR Lyrae variables

Infrared and optical absolute magnitudes are derived for the type II Cepheids kappa Pav and VY Pyx using revised Hipparcos parallaxes and for kappa Pav, V553 Cen and SW Tau from pulsational parallaxes. Revised Hipparcos and HST parallaxes for RR Lyrae agree satisfactorily and are combined in deriving absolute magnitudes. Phase-corrected J, H and K-s mags are given for 142 Hipparcos RR Lyraes based on Two-Micron All-Sky Survey observations. Pulsation and trigonometrical parallaxes for classical Cepheids are compared to establish the best value for the projection factor (p) used in pulsational analyses. The M-V of RR Lyrae itself is 0.16 +/- 0.12 mag brighter than predicted from an M-V-[Fe/H] relation based on RR Lyrae stars in the Large Magellanic Cloud (LMC) at a modulus of 18.39 +/- 0.05 as found from classical Cepheids. This is consistent with the prediction of Catelan & Cortes that it is overluminous for its metallicity. The M-Ks results for the metal- and carbonrich Galactic disc stars, V553 Cen and SW Tau, each with small internal errors (+/- 0.08 mag) have a mean deviation of only 0.02 mag from the period - luminosity (PL) relation established by Matsunaga et al. for type II Cepheids in globular clusters and with a zero-point based on the same LMC-scale. Comparing directly the luminosities of these two stars with published data on type II Cepheids in the LMC and in the Galactic bulge leads to an LMC modulus of 18.37 +/- 0.09 and a distance to the Galactic Centre of R-0 = 7.64 +/- 0.21 kpc. The data for VY Pyx agree with these results within the uncertainties set by its parallax. Evidence is presented that kappa Pav may have a close companion and possible implications of this are discussed. If the pulsational parallax of this star is incorporated in the analyses, the distance scales just discussed will be increased by similar to 0.15 +/- 0.15 mag. V553 Cen and SW Tau show that at optical wavelengths PL relations are wider for field stars than for those in globular clusters. This is probably due to a narrower range of masses in the latter case.