(SFR) in LBGs. First the SFR can be estimated from the UV continuum luminosity using Kennicut (1998): $SFR_{UV}= 1.4 \times 10^{-28}  L_\nu  M_\odot yr^{-1} $, where $L_\nu$ is the luminosity at rest-frame 1400 Åin units of $erg s^{-1} hz^{-1}$. This relation assumes a $10^8$ years timescale for a galaxy to reach the full UV luminosity, so for the youngest objects the conversion could underestimate the true SFR. The UV emission is very sensitive to the presence of dust and can be attenuated even by small amounts: we correct for this using the slope of the UV rest-frame continuum determined from the V and I band data and assuming a Calzetti law.\
A second value is given by the SED fit output $SFR_{SED}$. This clearly depends on the model assumed for the star formation history, in our case the exponentially declining star formation rate with e-folding time $\tau$.\
The median values for the 2 samples obtained for the SFRs in both ways are reported in Table 1. We find no significant differences between  and  in agreement with P07 for z$\sim $ 4 sources.\
A commonly used measure to assess the importance of the current episode of star formation to the buildup 