Period-Color and Amplitude-Color Relations for RR Lyraes
  We use published OGLE LMC/SMC data to present comprehensive Period-Color (PC)
and Amplitude-Color (AC) relations for both fundamental and overtone stars. For
fundamental mode stars, we confirm earlier work that the minimum light
extinction corrected PC relation in V-I has a shallow slope but with
considerable scatter (LMC: $[0.093 \pm 0.019]$ with a standard deviation about
this line of 0.116, SMC: $[0.055\pm0.058]$ with a standard deviation about this
line of 0.099). We note the high scatter about this line for both the LMC and
SMC: either there is some source of uncertainty in extinction or some other
physical parameter is responsible for this dispersion. We compare with previous
results and discuss some possible causes for this scatter. In contrast, RRc
overtone stars do not obey a flat PC relation at minimum light (LMC: $[0.604
\pm 0.041]$ with a standard deviation about this line of 0.109, SMC: $[0.472
\pm 0.265]$ with a standard deviation about this line of 0.091). The fact that
fundamental mode RR Lyrae stars obey a flat relation at minimum light and
overtone RR Lyrae stars do not is consistent with the interaction of the
stellar photosphere and hydrogen ionization front. We compare these results
with PC relations for fundamental and first overtone Cepheids. The fact that
the PC relations change significantly as a function of phase indicates strongly
that Cepheid and RR Lyrae relations can only be understood at mean light when
their properties as a function of phase are determined.
