To estimate interstellar extinction in the clusters, we have used (U-B, B-V) colour diagram. Since a well defined blue envelope of the MS can be easily identified in the CMDs, we have considered only those stars as probable MS stars which lie between the blue envelope and a red envelope, obtained by shifting the MS by in (B-V) colour. Probable main-sequence (MS) stars have been grouped in (B-V) colour bins of 0.1 mag. The central value of each bin and the mean value (U-B) is plotted in Figs. 9 (click here) and 10 (click here) for Be 64 and Be 69 respectively. The vertical bars are the standard errors. We find a mean reddening of for Be 64 and . for Be 69. The intrinsic ZAMS given by Mermilliod (1981) was used and the slope of E(U-B)/E(B-V) was taken to be equal to 0.72 (Johnson & Morgan 1953).
Figure 9: The colour - colour diagram for the probable main sequence stars in the cluster Be 64
Figure 10: The colour - colour diagram for the probable main sequence stars in the cluster Be 69
Figure 11: The comparison of evolutionary model of VandenBerg (1985) with the CMDs of Be 64. The contribution of probable foreground stars have been removed
Figure 12: Same as Fig. 11 but for Be 69
Figure 13: The comparison of theoretical isochrones of Bertelli (1994) with the CMDs of Be 64
Figure 14: Same as Fig. 13 but for Be 69