(1999). According to the analysis by Grundahl et al. (1999) M13 may be younger than M3 by $0.7\pm0.2$ Gyr, while for Rey et al. (2001) it is older by about $2$ Gyr.\
Ages reported by other authors for the clusters of our analysis agree quite well within the error bars. Salaris & Weiss (2002) found a typical difference smaller than $\Delta t=1$ Gyr independently of the metallicity scale used. Ages by D10 obtained using the isochrone-fitting method are basically identical; in fact t=$(12.5\pm0.5)$ Gyr and t=$(13.0\pm0.5)$ Gyr for M3 and M13 respectively, while M79 is not in their sample. The same results were obtained by Marin-Franch et al. (2009). For G10 instead, M13 is older than M3 and M79 of about $\Delta t =$ 1.75 Gyr.\
It is worth noticing that even moderate differences of the average Y among these three clusters do not affect appreciably the results of the methods employed to determine their relative ages (see, e.g., Marin-Franch et al. 2010).\
We investigated any possible age difference by comparing the ($m_{F555W}, m_{F336W}-m_{F555W}$) CMDs of the three clusters of our analysis.

![As in Fig. \[m13m79\] but for M3 and M13.[]{data-label="m13m3"}](f4.ps){width="98mm"}

We used M13 as reference and shifted M79 and 