2 Observations and reductions

Our spectroscopic and photometric observations were performed at the 1.6 m (B&C) and 60 cm (Zeiss) telescopes of LNA, respectively.

2.1 Spectroscopic observations

Spectroscopic observations were obtained during six nights of March 1995 (over a total time span of eight nights, see Table 1) with the coudé spectrograph of the 1.6 m telescope. A EMI CCD camera (1152$\times$770 pixels) was used. Spectra were taken with a grating of 1 800 l/mm (first inverse order) centered on the HeI $\lambda$667.8 nm line. This arrangement yields a spectral resolution of $R\simeq60~000$ and a wavelength range of $\simeq$ 89 Å. The signal-to-noise ratio was typically $\simeq$ 250 for exposure times between 300 and 900 s. The usual bias, flat field and Th-Ar comparison lamp spectra were taken during each night. Observations were reduced with IRAF using standard techniques for two-dimensional CCD data.

  

Table 1: Log of observations at LNA

*Strömgren b filter.

2.2 Photometric observations

Ten nights of b (Strömgren) observations of $\eta$Cen were obtained in May, July and August 1993 (Table 1) with a two-channel photometer attached to the 60 cm Zeiss reflector of Laboratório Nacional de Astrofísica (LNA). The mean integration time was $\simeq$ 5  s. Only the stellar channel of the photometer is refrigerated and equipped with a b filter; the second channel is simply used to monitor a comparison star in white light. As we were only interested in searching for periodic variations with timescales 1 hour, data reduction consisted in fitting a low order polynomial to the stellar and comparison light curves for each night, and in determining the ratio variable flux/comparison flux for each Be star measurement. Frequency analysis was performed for the time series formed by these ratios.