2 Observations and data reduction

Spectroscopic observations of BD  $+60^{\circ }$ 2522 were collected during several observing campaigns between August 1996 and September 2002 at the Observatoire de Haute-Provence (OHP).

Our observations were obtained with the Aurélie spectrograph fed by the 1.52 m telescope at OHP (Gillet et al. 1994). Until 1999, Aurélie was equipped with a Thomson TH7832 linear array with a pixel size of 13 $\mu$m. From 2000 on, the detector of the Aurélie instrument was replaced by a 2048$\times$1024 CCD EEV 42-20#3, with a pixel size of 13.5 $\mu$m squared.

Table 1 provides an overview of the various instrumental configurations. All the data were reduced in the standard way using the MIDAS package developed at ESO. The spectra were normalized using properly chosen continuum windows.

Figure 1: Mean spectrum of BD  $+60^{\circ }$ 2522 as observed in August 1996.

Table 1 also yields some information about the sampling of the various observing campaigns. For each campaign, $\Delta T$ gives the total time elapsed between our first and our last observation, while N is the total number of observations. $\overline {\Delta t}$ provides the average time interval between two consecutive exposures during the same night. In light of the Fourier analysis, Table 1 further lists the natural width of a peak in the power spectrum $\Delta~\nu_{\rm nat} = 1/\Delta T$, while $\nu_{\rm max} = 1/(2~\overline{\Delta t})$ provides a rough indication of the highest frequencies that could be sampled with our data set.