The color-magnitude diagrams ( CMDs ) of three intermediate-age LMC clusters , NGC 2173 , SL556 and NGC2155 are analyzed to determine their age and metallicity basing on Padova stellar models .
Synthetic CMDs are compared with cluster data .
The best match is obtained using two fitting functions based on star counts in the different bins of the cluster CMD .
Two different criteria are used .
One of them takes into account the uncertainties in the color of the red clump stars .
Given the uncertainties on the experimental values of the clusters metallicity , we provide a set of acceptable solutions .
They define the correspondent values of metallicity , age , reddening and distance modulus ( for the assumed IMF ) .
The comparison with Padova models suggests for NGC 2173 a prolonged star formation ( spanning a period of about 0.3 Gyr ) , beginning 1.7 Gyr and ending 1.4 Gyr ago .
The metallicity Z is in the range 0.0016 - 0.003 .
Contrary to what suggested for NGC 2173 , a period of extended star formation was not required to fit the SL 556 and the NGC 2155 observations .
For SL 556 an age of 2.0 Gyr is obtained .
The metallicity value is in the range 0.002 - 0.004 , depending on the adopted comparison criterium .
The derived age for NGC 2155 is 2.8 Gyr and its metallicity Z is in the range 0.002 - 0.003 .
The CMD features of this cluster suggest that a more efficient overshoot should be adopted in the evolutionary models .