The Bologna/ESO Search for Double Degenerates

Extract

In 1984 a program has been set up for a systematic search of double degenerates (DD) among spectroscopically confirmed white dwarfs. For each WD at least two CCD spectra are obtained, which are then cross-correlated in order to get a radial velocity difference, if any. For this purpose we have used the ESO Faint Object Spectrograph and Camera (EFOSC) attached at the ESO 3.6m telescope. Exposure times are typically in the range 5–15 min, and the typical error in the radial velocity difference (Δ_υx) is ∼ 20 km s⁻¹.

Until now we had two observing runs, respectively in September 1985 and January 1988, for a total of 5 usable nights. For 20 WDs we have obtained more than one spectrum. Among them we have singled out two DD candidates. The first object (WD0957–666) has shown Δ_υr values up to ∼ 220 km s⁻¹ and three other spectra were kindly taken for us in April 1988 by Stefano Cristiani. These also showed Δ_υr,’s up to ∼ 140 km s⁻¹. Another object (WD0954–710) exhibits a Δ_υr ≃ 70 km s⁻¹. The two DD candidates will be observed again in the next observing run in order to confirm their binarity and in case determine the periods.