Appendix

The NTT images were calibrated on the basis of I$_{\rm KC}$=11.65$^{\rm m}$ for LTT 3218 (Landolt 1992, Hamuy et al. 1992) and I$_{\rm KC}$=15.72$^{\rm m}$ and 15.02$^{\rm m}$ for stars o respectively m in the E5 region (Graham 1982). The EFOSC images were calibrated based on B=12.29$^{\rm m}$ and I$_{\rm KC}$= 11.12$^{\rm m}$ for LTT 6248 in the case of PKS 1353-341 and on B=10.88$^{\rm m}$ and I$_{\rm KC}$=11.27$^{\rm m}$ for EG 274 for the other two (Landolt 1992, Hamuy et al. 1992). The colour equation determined by Benetti (1996) has been taken into account. The atmospheric extinction coefficients are those measured by the Geneva observatory Photometric Group.

The spectra were calibrated with W485A (Oke 1974).