  A compilation of photometric time-series data of NGC 4147 spanning 58 years,
enabled to study the secular behaviour of the pulsation period in the
population of RR Lyrae stars in the cluster. The traditional O-C diagram
approach was employed. Three stars display a significant period decrease and
two a comparable period increase. Nevertheless, the median value of the whole
sample is nearly zero, which seems consistent with theoretical expectations.
The limitations of empirical determinations of secular period changes in
globular clusters to corroborate evolutionary theoretical predictions are
discussed.
