SIMBAD references

A compilation of photometric time-series data of NGC 4147 spanning 58 years, enabled us to study the secular behavior of the pulsation period in the population of RR Lyr stars in this 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.