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can't say I know that much about variable stars, been looking at rr lyrae types lately and they are the senior citizens or the celestial set. population 2 stars meaning old, they are running on low hydrogen reserves. This causes them to cool down, and in so doing, they contract, which causes an increase in internal temp. They must then expand in order to radiate the incresed heat energy or else explode. When they expand, they loose the excess heat because there is insufficient fuel to maintain that higher level of radiation. This in turn cuases them to again shrink, repeating the cycle. When the star expands and contracts its luminosity also cycles, but I think it is not what would seem logical at first. The larger size is also the brightest, not the coolest.
for a better explanation google 'rr lyrae variables'
Doug
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