FG SAGITTAE - NO HOT COMPANION

The extraordinary object, FG Sge, was observed in the low-dispersion mode of the International Ultraviolet Explorer (IUE) for a 407 min exposure with the SWP camera with the aim of determining if a long exposure might reveal the presence of an unseen hot companion to the optically bright variable This new observation shows no evidence for such a companion in the 1200-2000 Å range of the SWP camera and supports the contention that FG Sge underwent a helium shell flash during the past century, and that the surrounding nebula, He 1-5 (= PK 60 - 7°1) is a nebula of fossil ionization. The current observed ionization presumably originated due to photoionization when FG Sge was much hotter than its present Teff < 5000 K. However, constraints posed by the IUE detection limit, the observed Hβ flux, and adopted radii for white dwarfs still allow the possibility of a putative companion. If a hot companion were to exist, and if a stellar mass of ∼0.6ℳ⊙ is assumed, it must have 100 000 > Teff > 70 000 K and 6.7 < log(g) < 8.0. Analysis of IUE LWP archival data also shows that in late 1988 the spectral type of FG Sge was G8-K0, in agreement with optical data.