9 Colour-magnitude diagrams

The observed colour-magnitudes diagrams are seen in Fig. 9. Note that the V magnitude and the b-y index have been corrected for interstellar extinction ( $A_V = 4.3\ E(b-y)$). In each diagram 606 (NGC 6134) and 164 (NGC 3680) stars are plotted. As the metallicity of the clusters are comparable it is evident that NGC 6134 is younger than NGC 3680, i.e. no A type stars are seen in the CMD of NGC 3680.

Figure 9: Observed colour-magnitude diagrams of NGC 6134 and NGC 3680. In the CMD of NGC 6134 the boxes mark the location of the six known $\delta$ Scuti stars and the dashed curves show the location of the instability strip according to observations of field stars by Rodriguez et al. (1996). The diamond marks the blue straggler found by Frandsen et al. (1996). In the CMD of NGC 3680 the boxes mark the stars that are cluster members according to Nordström et al. (1997)