Deep CCD photometry in the uvby - H _ { \beta } intermediate-band system is presented for the cluster NGC 2548 ( M 48 ) .
A complete membership analysis based on astrometric and photometric criteria is applied .
The photometric analysis of a selected sample of stars yields a reddening value of E ( b - y ) = 0.06 \pm 0.03 , a distance modulus of V _ { 0 } - M _ { V } = 9.3 \pm 0.5 ( 725 pc ) and a metallicity of [ Fe/H ] = - 0.24 \pm 0.27 .
Through isochrone fitting we find an age of \log t = 8.6 \pm 0.1 ( 400 Myr ) .
Our optical photometry and JHK from 2MASS are combined to derive effective temperatures of cluster member stars .
The effective temperature distribution along the main sequence of the cluster shows several gaps .
A test to study the significance of these gaps in the main sequence of the HR diagram has been applied .
The method is also applied to several other open clusters ( Pleiades , Hyades , NGC 1817 and M 67 ) to construct a sequence of metallicities and ages .
The comparison of the results of each cluster gives four gaps with high significance ( one of them , centred at 4900 K , has not been previously reported ) .