HM Sagittae is one of the most unusual objects in the Galaxy for it displays activity in every spectral band from x-ray to radio. Its present variable-star designation was given after the discovery of its optical brightening from 16m to 12m between April and September 1975 (Dokuchaeva and Balazs 1976). It was soon found to have a rich emissionline spectrum similar to that of a planetary nebula (Stover and Sivertsen 1977). Post-brightening monitoring of the object by Ciatti, Mammano and Vittone (1977, 1978) found the B and V magnitudes to be variable with amplitudes of at least one magnitude. Evidence for increasing excitation was found by Ciatti, Mammano and Vittone (1979) with Hell 4686 emerging in October 1978. Wolf-Rayet features of velocities up to 2000 km s-1 have also been seen (Belyakina, Gershberg and Shakhovskaya 1978, 1979; Brown et al. 1978; Ciatti et al. 1978; Wallerstein 1978; Allen 1980; Andrillat and Swings 1982). Analysis of the forbidden line ratios gives an estimated nebula density of 106-107 cm-3 and a gas temperature >104K (Ciatti et al. 1977; Arkhipova and Dokuchaeva 1978; Davidson, Humphreys and Merrill 1979; Arkhipova, Dokuchaeva and Esipov 1979).
Discovery of Extended X-Ray Emission from the Planetary Nebula NGC 7027 by the [ITAL]Chandra X-Ray Observatory[/ITAL]
