Complete sample of Ap stars in the Hertzsprung-Russell diagram. Stars denote the known rapidly oscillating Ap stars, while circles denote the Ap stars for which pulsations have not been detected yet. The dashed blue lines denote the blue and red edges of the theoretical instability strip. The thin dash-dotted lines correspond to evolutionary tracks for masses ranging from 1.4 M_⊙ to 2.5 M_⊙ and the thick black line denotes the ZAMS. The colour-coding gives the stellar magnetic field strength Bs. The numbers close to the symbols refer to Table 4.