SIMBAD references

Infrared and optical absolute magnitudes are derived for the type II Cepheids κ Pav and VY Pyx using revised Hipparcos parallaxes and for κ 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 & Cortés that it is overluminous for its metallicity. The results for the metal- and carbon-rich 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 R₀= 7.64±0.21kpc. The data for VY Pyx agree with these results within the uncertainties set by its parallax. Evidence is presented that κ 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 ∼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.