Periods against I magnitudes for the CWs and RVs found in NGC 6388 and NGC 6441. Green circles are CWAs, cyan squares are CWBs, yellow triangles are RVs, and magenta diamonds are ACs. The black star and black cross in the NGC 6441 plot are the variable stars V6 and V132, respectively, for which mean I magnitudes and periods are taken from P03. The solid lines are derived based on CWAs and CWBs from this study only and have R²-values of 0.991 and 0.848 for NGC 6388 and NGC 6441, respectively. The dashed lines are the relations found in P03 (see Sect. 4.1 for details).