SIMBAD references

The period of an RR Lyrae variable can be determined far more accurately than any other characteristic of the star, in principle affording an exquisitely sensitive means of measuring tiny changes in the density of the star, and thus its evolution through the instability strip. Although much effort has been devoted to determining RR Lyrae period changes in the century since the class of RR Lyrae variables was defined, period change "noise" has complicated the measurement of evolution in RR Lyrae stars. These noisy period changes are themselves of interest, however, and they may reveal hitherto unsuspected attributes of RR Lyrae stars. The status of the problem is reviewed.