What is the nature of the spectroscopic companion of the early B star λ Scorpii?

The B star lambda 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 Berghofer 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 lambda 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 lambda 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 lambda Sco shows significant short-term variations on a 20% level. A period-folding search carried out to further investigate the EUV light curve of lambda 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 sigma feature at 4.7 cycles days(-1), which is close to the main pulsation frequency of the B 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 lambda Sco shows two broader dips of 30% intensity loss at phases phi = 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 lambda 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.