2 Observations and data reduction

Observations of the Be II doublet at 3131 Å were carried out during 5 different observing runs. Two of them made use of the UVES spectrograph, at the VLT/UT2 Kueyen telescope (ESO, Chile). The obtained spectra have a resolution $R\equiv\lambda/\Delta\lambda$ of around 70 000 for one of the runs (66.C-0116 A) and 50 000 for the other (66.D-0284 A), and the S/N ratios in the region around 3131 Å varying between 30 and 250 (from now on we will refer to runs UVES(A)/(B), respectively). UVES spectra were complemented with data obtained using the UES spectrograph (R=55 000) at the 4.2-m William Herschel Telescope (WHT) and, in two different observing runs (A/B), with the IACUB echelle spectrograph (R=33 000) at the 2.6-m Nordic Optical Telescope (NOT), both at the Observatorio del Roque de los Muchachos (La Palma). The observations, together with the corresponding observing run and S/N ratios obtained are listed in Table 1.

Data reduction was done using IRAF tools in the echelle package. Standard background correction, flat-field, and extraction procedures were used. For the UVES(A) run and for the UES data, the wavelength calibration was done using a ThAr lamp spectrum taken during the same night. For the UVES(B) and IACUB runs the wavelength calibration was done using photospheric lines in the region of interest. The final linear dispersions for the UVES(A), UVES(B), IACUB(A), IACUB(B), and UES spectra were 30, 17, 35, 35 and 17 mÅ pixel^-1, respectively, with rms values in the range 4 $\times10^{-4}$ to 8 $\times10^{-3}$ Å.