A Preliminary Calibration of the RR Lyrae Period-Luminosity Relation at
  Mid-Infrared Wavelengths: WISE Data
  Using time-resolved, mid-infrared data from WISE and geometric parallaxes
from HST for four Galactic RR Lyrae variables, we derive the following
Population II Period-Luminosity (PL) relations for the WISE [W1], [W2] and [W3]
bands at 3.4, 4.6 & 12 um, respectively:
  M[W1] = -2.44 (+/- 0.95) x logP - 1.26 (+/- 0.25) sigma = 0.10
  M[W2] = -2.55 (+/- 0.89) x logP - 1.29 (+/- 0.23) sigma = 0.10
  M[W3] = -2.58 (+/- 0.97) x logP - 1.32 (+/- 0.25) sigma = 0.10
  The slopes and the scatter around the fits are consistent with a smooth
extrapolation of those same quantities from previously-published K-band
observations at 2.2 um, where the asymptotic (long-wavelength) behavior is
consistent with a Period-Radius relation having a slope of 0.5. No obvious
correlation with metallicity (spanning 0.4 dex in [Fe/H]) is found in the
residuals of the four calibrating RR Lyrae stars about the mean PL regression
line.
