  Deep CCD photometry in the uvby-Hbeta 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\pm0.03, a
distance modulus of V_0-M_V=9.3\pm0.5 (725 pc) and a metallicity of [Fe/H]=
-0.24\pm0.27. Through isochrone fitting we find an age of log t = 8.6\pm0.1
(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).
