2003A&A...399..681G

We report the first far-ultraviolet spectrum of the emission line spectroscopic binary KQ Pup (M2Iab+B0Ve), obtained with FUSE shortly after conjunction at orbital phase Φ=0.13. The spectrum presents a sharp flux cutoff at 1040Å; longwards, it is dominated by a large amount of resonant and low excitation transitions of neutral and singly ionized species, probably mostly of circumsystem origin, and by the Lyman absorption bands of H₂, whose strength corresponds to an interstellar H₂ column density of ∼2x10²⁰cm^–2, indicating a fraction of molecular hydrogen 2N(H₂)/[N(HI)+2N(H₂)] of ∼0.4. An N(HI)/E_B–V ratio of 4.3x10²¹ is derived from the IUE spectra. The long-term UV monitoring of KQ Pup with IUE, HST and FUSE reveals a large decrease of the far-UV flux since orbital phase 0.82 and occupying more than 0.3 of the orbital period. This ``shell'' episode is attributed to line absorption and to Lyα Rayleigh scattering due to an extended dense cool envelope in the line of sight of the B star.