Abstract
The B star λ Sco is known to be a spectroscopic binary system. The companion, which is in a short periodic orbit with the B star, is so far unknown. X-ray observations with ROSAT by Berghöfer et al. in 1997 have shown a super-soft X-ray excess, which is unusual for B stars of spectral type B1.5 IV. Here we present an analysis of our long Extreme Ultraviolet Explorer (EUVE) observation of λ Sco. Based on these data and all available X-ray observations of this star, we constrain the physical parameters of the companion. As long as no other explanation is available for the EUV/soft X-ray excess, the spectroscopic companion of λ Sco is most likely an ultramassive white dwarf. The primary B star is thus the most massive star known to have a white dwarf companion. Such a stellar system can have evolved only by mass transfer. Stellar evolution scenarios predict the existence of such binary systems, which are expected to be precursors of the ultrasoft X-ray sources and which finally explode in a supernova Type Ia. The EUV light curve of λ Sco shows significant short-term variations on a 20% level. A period-folding search carried out to further investigate the EUV light curve of λ Sco does not provide clear evidence for any periodicity present in the data. It is worthwhile to mention that the analysis of variance periodogram shows a 2 σ feature at 4.7 cycles days-1, which is close to the main pulsation frequency of the β Cep-type B star. Further observations have to confirm the existence of such a periodicity in the EUV light curve. Furthermore, when folded with the orbital period, the EUV light curve of λ Sco shows two broader dips of 30% intensity loss at phases ϕ = 0.56 and 0.11. At these two phases, the stars pass each other in the line of sight. We discuss these features in the EUV light curve of λ Sco in terms of orbital dependent changes in the absorption column of the primary's wind along the line of sight toward the white dwarf companion.
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