On the pulsation parallax of the variable star RR Lyr

We show that a straightforward application of the predicted relation connecting the absolute K magnitude of fundamental RR Lyrae variables to their period and metal content, together with current evolutionary predictions on the mass and luminosity of Horizontal Branch stars, supply a distance estimate to the prototype star RR Lyr in close agreement with the recent parallax determination by HST (Benedict et al. 2002), largely unaffected by the interstellar extinction correction. Adopting <A_V>=0.12±0.10 mag as a safe estimate of the extinction correction to RR Lyr, we obtain a “pulsation” parallax of 3.858±0.131 mas which agrees quite well, and with a smaller formal error, with the HST measurement (3.82±0.20 mas) and with the weighted average of HST, Hipparcos, and ground-based determinations (3.87±0.19 mas). This result suggests that near-infrared magnitudes and pulsation models could constrain the distance to well-studied RR Lyrae stars, both in the field and in globular clusters, with an accuracy better than current direct trigonometric measurements.