SIMBAD references

We have used data from the literature to reexamine the period changes of RR Lyrae variables in seven globular clusters. There is a tendency for the ore extreme rates of period change to be found in RRab rather than RRcd stars. Particularly absent from the RRcd star sample are stars showing large rates of period decrease, a result which may in part reflect the small proportion of RRcd variables in Oosterhoff type I clusters. The apparent coupling of period change to pulsation mode is not easily interpreted as an evolutionary effect, and appears to be without adequate explanation.