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Astron. Astrophys. 347, 69-76 (1999)

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2. Observations and data reductions

2.1. Strömgren photometry

Strömgren photometry (used in the determination of stellar effective temperatures) was not available for several of the low gravity stars from the Saffer et al. (1997) sample. Observations were made using the 1.0-m Jacobus Kapteyn Telescope (JKT) on the island of La Palma on the nights of 8th and 9th December 1996, and employed the uvby filter set along with the Tek4 CCD. As the primary standards generally used in Strömgren photometry are too bright for this system, nearby stars observed by Stetson (1991) were used to standardize the instrumental magnitudes. Three exposures were obtained for both our targets and standards in each band-pass. This led to a photometric accuracy of [FORMULA]0.01 magnitudes in all filters, and the results for PG 0855+294, PG 0914+001 and PG 0955+291 are presented in Table 1.


Table 1. Observational details of the programme stars.
(1) Green et al. (1986); (2) Wesemael et al. (1992); (3) this paper; (4) Tobin (1986); (5) Moehler et al. (1990); (6) Allard et al. (1994).

2.2. High-resolution spectroscopy

Programme stars for high-resolution follow-up were selected from the sample presented in Saffer et al. (1997) and are detailed in Table 1, where photometric data and references are also given. The spectroscopic data were obtained using a variety of telescopes and instruments, including the William Herschel Telescope 4.0-m + UES/ ISIS spectrographs, the Mayall 4.0-m + échelle spectrograph and the Anglo-Australian Telescope 3.9-m + UCLES spectrograph. All observations employed CCDs as detectors. Reduction of the 2-dimensional CCD images to wavelength calibrated spectra was performed using tasks within the IRAF environment (Tody 1986). Standard procedures were followed, viz. overscan correction, trimming of the data section, flat-fielding, sky subtraction, extraction of the stellar spectra and wavelength calibration. Further details of the observations and reductions appear in Paper I, to which we refer the reader.

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© European Southern Observatory (ESO) 1999

Online publication: June 18, 1999