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Astron. Astrophys. 329, L13-L16 (1998)

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2. Observations

BPM 24754 is listed in the McCook & Sion (1987) catalogue as WD 1714-547 with 1950 coordinates [FORMULA] and [FORMULA], with [FORMULA], [FORMULA], [FORMULA], and a spectral type of DA7. Its finding chart can be found in Luyten (1949) under the name L 269-72. Time series photometry and spectroscopic observations of BPM 24754 are reported here.

The time series photometry was obtained at the Laboratório Nacional de Astrofsica (LNA)/CNPq, Brazil, in 1995 August and 1996 July using the two-star Texas Photometer (Nather 1973) attached to the 1.6 m telescope. In both channels we used Hamamatsu blue-sensitive photomultipliers with no filter to maximize the signal-to-noise ratio. A total of 21.6 hours of data during 7 nights were gathered on BPM 24754. The journal of observations is presented in Table 1. Sky measurements were made about every [FORMULA]  min in both channels. The standard data reduction as described by Nather et al. (1990) was applied to light curves: sky subtraction by interpolation, extinction correction, and normalization to mean intensity. The resulting time series are in units of fractional intensity (mi) relative to the mean intensity of the star (mi [FORMULA], i.e., 1 mi corresponds to variation of 100%.). The seven time series are plotted in Fig. 1.


Table 1. Journal of observations at LNA

[FIGURE] Fig. 1. Time series. Fractional intensity vs. time in minutes.

The spectroscopic observations were carried out at the Cerro Tololo Inter-American Observatory (CTIO) in 1995 August. We used a Cassegrain Spectrograph on the 1.5 m telescope with a 300  [FORMULA] /mm grating and a 250 arcsec-wide slit. This configuration provided a spectral resolution of [FORMULA]  Å. Because high signal-to-noise spectroscopy is necessary to determine the atmospheric parameters with a precision of 200 - 300 K in [FORMULA], we observed the star for 2 hours in 6 exposures of 20 min. After flux calibrating each frame, the spectra were co-added and the resulting spectrum is shown in Fig. 2.

[FIGURE] Fig. 2. Top: optical spectrum. Bottom: line profile fitting. Comparison of the observed optical lines with the closest synthetic spectrum (dotted). Both spectra are normalized to continuum.

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© European Southern Observatory (ESO) 1998

Online publication: November 24, 1997