Using the extinction coefficients RF606W = 2.8192 and RF814W = 1.8552 (Cardelli et al. 1989; ODonnell 1994) and assuming for M3 a distance modulus of (m M)0 = 15.00 0.04 (Ferraro et al. 1999b; Dalessandro et al. 2013) and a colour excess E(BV) = 0.01 (Ferraro et al. 1999b), we found (m M)0 = 18.58 and E(B V) = 0.08 for NGC 1835. The same results are obtained by adopting M13 as a reference and thus assuming its distance modulus and reddening, (m M)0 = 14.38 0.05 and E(B V) = 0.02 (Ferraro et al. 1999b). These findings are fully consistent with previous estimates in the literature: in fact, a reddening value of E(B V) = 0.08 0.02 and a distance modulus ranging between 18.43 0.12 and 18.65 0.16 are quoted in Olsen et al. (1998).

To determine the absolute age of NGC 1835 we adopted the isochrone fitting technique. This consists in comparing the cluster CMD with a set of isochrones of different ages to identify the one that best reproduces the observed evolutionary sequences. We extracted isochrones from two different databases, BaSTI (Pietrinferni et al. 2021) and PARSEC (Bressan et al. 2012; Chen et al. 2015). BaSTI isochrones have been computed for [Fe/H]= 1.7, an element abundance [/Fe]= +0.4 (Mucciarelli et al. 2021), and a standard helium abundance Y = 0.248. Similarly, we downloaded solar scaled PARSEC isochrones with a total metallicity [M/H] = = 1.4, that corresponds to [/Fe]= +0.4 and [Fe/H]= 1.7. Both datasets were downloaded for a suitable range of ages (between 10 Gyr and 15 Gyr in steps of 0.5 Gyr). They have been superposed to the observed CMD by adopting the distance modulus and the reddening values estimated above. A small color offset of (mF606W mF814W) = 0.015 mag (lower than the typical

Using 2 tests, we compared the isochrones and the observed data in the most age-sensitive region of the CMD (namely, the MS-TO and the sub-giant branch, in a magnitude range 21.8 < mF606W < 23.0). For a maximum photometric quality of the data and to avoid severe contamination from the LMC field, we limited the analysis to an annular region between 15 and 26 from the cluster center.

The results obtained for the two considered models are shown in Figure 6. The left panels depict the trend of the 2 values for different ages and show that, for both the adopted model, the minimum 2 value (i.e., the best-fit solution) is found at an age of 12.5 Gyr, with a conservative uncertainty of 1 Gyr. The right panels of Fig. 6 show the isochrones corresponding to the best-fit ages (solid red line) along with the uncertainty ranges (dashed lines). Article number, page 5 of 10