We report the first detection of the [Ar VI] 4.53 µm and [Ne VI] 7.65 µm lines in the spectrum of NGC 7027. When compared with older observations it is clear that the [Ar VI] line and possibly also other lines have increased in strength since 1981. We argue that a likely explanation for this variability is a change in the spectral energy distribution of the central star, possibly an increase in effective temperature. However, this result needs to be confirmed by further observations.

We also report a non-detection of the [O IV] 25.9 µm line and the first detection of the [Na III] 7.32 µm line in the spectrum of NGC 6543. The non-detection is not expected based on a blackbody approximation for the spectrum of the central star. The ionization threshold for O³⁺ is just beyond the He II limit, and the absence of this line shows that the stellar flux drops at least by a factor 350 at the He II limit. Modeling the [O IV] line may prove to be a valuable test for atmosphere models.

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Peter van Hoof
Royal Observatory of Belgium
Ringlaan 3
1180 Brussel
Belgium

email: p DOT vanhoof AT oma DOT be