7 Spectrum of the underlying star

Since we do not observe absorption lines before 5600 Å, we can only use the few lines in the yellow and the red region. We can identify four lines with Al II(6066), S II(6102), N II(6632) and C II(6800). An early spectral type around B0 is suggested, which would be in line with a shell which has He I in emission. The evidence is not very strong however. Leibowitz (1977) proposed on theoretical grounds a type $\rm O9-O9.5$ for the underlying star. The UBV colors measured by Martel & Gravina (1985a): U-B=-0.20, B-V=+0.42 would lead to a B4 star with E(B-V)=0.60. With such a value of the color excess, an extinction of about 1.8 would be expected, which leads with M=-1.4 and V=11.6 to 1700 pc.

Contrary to the opinion of Ciatti et al. (1974) we do not observe the presence of a late type companion.