  Acquiring near-infrared K-band (2.2 um) photometry for RR Lyrae variables in
globular clusters and nearby galaxies is advantageous since the resulting
distances are less impacted by reddening and metallicity. However, K-band
photometry for RR Lyrae variables in M5, Reticulum, M92, omega Cen, and M15
display clustercentric trends. HST ACS data imply that multiple stars in close
proximity to RR Lyrae variables located near the cluster core, where the
stellar density increases markedly, are generally unresolved in ground-based
images. RR Lyrae variables near the cluster cores appear to suffer from
photometric contamination, thereby yielding underestimated cluster distances
and biased ages. The impact is particularly pernicious since the contamination
propagates a systematic uncertainty into the distance scale, and hinders the
quest for precision cosmology. The clustercentric trends are probably
unassociated with variations in chemical composition since an empirical K-band
period-magnitude relation inferred from Araucaria/VLT data for RR Lyrae
variables in the Sculptor dSph exhibits a negligible metallicity dependence:
(0.059+-0.095)[Fe/H], a finding that supports prior observational results. A
future multi-epoch high-resolution near-infrared survey, analogous to the
optical HST ACS Galactic Globular Cluster Survey, may be employed to establish
K-band photometry for the contaminating stars discussed here.
