by Cohen & Melendez (2005) M3 and M13 are slightly more metal-rich, i.e. \[Fe/H\]=$-1.39\pm0.02$ and \[Fe/H\]=$-1.50\pm0.01$. In addition in this case the observed metallicity difference ($\Delta$\[Fe/H\]$\sim0.1$) is larger than the uncertainties.\

![Upper panel. Optical CMD of M79 super-imposed to M13 after correcting for differences in distance modulus and reddening. Bottom panels. Zoomed view of the TO region. On the left, the observed CMDs are shown with two isochrones from the BaSTI database with $t=12$ Gyr and $t=13.5$ Gyr. The right panel shows synthetic CMDs obtained from the adopted isochrones and the appropriate photometric errors.[]{data-label="m13m79"}](f3.ps){width="98mm"}

On the contrary the determination of their ages is a more debated argument. In literature there is a quite general consensus that M3 and M79 are old and coeval GGCs. There is instead more uncertainty about the age of M13. Ferraro et al. (1997) put constraints on the possible difference in age between M13 and M3 by comparing their ridge mean lines obtained with the optical CMDs used in this work, and theoretical models by Dorman et al. (1996). The age difference resulted to be smaller than $\Delta t=1-1.5$ Gyr. The same conclusions have been reached also by Johnson & Bolte (1998) and Rosenberg et al. 