First far-UV observations of KQ Puppis with FUSE

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 0.13. The spectrum presents a sharp flux cutoff at 104 nm; 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 molecular hydrogen H_2, whose strength corresponds to an interstellar H_2 column density of about 2x10^20 cm^-2, indicating a fraction of molecular hydrogen 2N(H_2)/[N(HI)+2N(H_2)] of about 0.4. An N(HI)/E(B-V) ratio of 4.3x10^21 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 orbital phase. This `shell' episode is attributed to line absorption and to Ly_alpha Rayleigh scattering due to an extended dense cool envelope in the line of sight of the B star.