On Wednesday, November 19, 2014 3:15:50 AM UTC-5, Richard D. Saam wrote:

What is the theoretical source of such an observed consistent period
that you allude to?

I wouldn't say that 0.320 +/- 0.047 days is a consistent period, but
here are a couple of factors that influence the observed distribution
of RR Lyrae periods:

1. There are instability strips in the HR (temperature - luminosity)
diagram where stars are unstable against pulsations. These are the
temperatures and densities where opacity increases with temperature
( http://en.wikipedia.org/wiki/Kappa_mechanism )

2. One must then determine if the evolutionary tracks of stars put
them in the instability strip for a significant amount of time.
Following the helium flash at the tip of the red giant branch, core
helium burning stars are located along the "horizontal branch" in the
HR diagram. Most RR Lyrae stars are located in the fairly narrow
intersection region of the horizontal branch with the instability
strip. The periods vary from the blue end to the red end of this
intersection region but the range.

[Mod. note: reformatted -- mjh]