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A compilation of photometric time-series data of NGC 4147 spanning 58 years,enabled to study the secular behaviour of the pulsation period in thepopulation of RR Lyrae stars in the cluster. The traditional O-C diagramapproach was employed. Three stars display a significant period decrease andtwo a comparable period increase. Nevertheless, the median value of the wholesample is nearly zero, which seems consistent with theoretical expectations.The limitations of empirical determinations of secular period changes inglobular clusters to corroborate evolutionary theoretical predictions arediscussed.

Secular period variations of RR Lyrae stars in NGC 4147