redshift uncertainty quoted there is $\sigma_z=0.0013$, obtained from a comparison between redshifts of the same objects obtained more than once with different set-ups.\
The spectra were brought to rest-frame and co-addition was performed adopting a sigma–clipping procedure to eliminate sky-subtraction residuals, zero-count pixels due to CCD gaps, and other spectral blemishes. After several tests, the best results were obtained when a single 3$\sigma$ clipping iteration was adopted. On average more than 90% of all galaxies in a given bin contribute to the stacked spectrum at any given wavelength.\
In Figure \[fig:spectra\] we present the composite spectrum of all $LBG_L$ and compare it to the $LBG_N$ spectrum, after normalizing the two spectra at 1280 Å.

![The stacked spectra of  (black line)and  (green line). Beside the obvious difference in the region, the different spectral slope and strength of the absorption lines are also clearly shown. []{data-label="fig:spectra"}](pentericcifig6.eps){width="9cm"}

Besides the obvious difference of the presence of bright Ly$\alpha$ in the spectrum of $LBG_L$, we note that the spectrum of $LBG_N$ is flatter than the spectrum of $LBG_L$, in agreement with the higher value of E(B-V) derived from the SED fit and indicating in  the presence of more dust than in the . Furthermore, in 