All Figures

$\begin{figure} \par\includegraphics[width=18cm,clip]{h4416f1.ps} \end{figure}$ Figure 1: The UV spectrum of N 66 at different epochs, showing the dramatic rise and fall of the stellar flux. Note that nebular emission strongly contaminates the IUE observations, but is effectively suppressed by the small apertures of the HST spectrographs.
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In the text

$\begin{figure} \par\includegraphics[width=18cm,clip]{h4416f2.ps} \end{figure}$ Figure 2: Spectrum of N 66 (continuous line) at epoch 1994.7, i.e. when the star was brightest, observed with IUE (UV) and CTIO (visual), after subtracting nebular continuum and nebular UV lines (see text). Superimposed is the synthetic model B scaled to a luminosity of $\log L/L_{\odot} = 5.43$ (dotted line). While the uppermost panel shows absolute fluxes, in the lower three panels the observed fluxes were divided by the model continuum.
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In the text

$\begin{figure} \par\includegraphics[width=18cm,clip]{h4416f3.ps} \end{figure}$ Figure 3: Spectrum of N 66 (continuous line) at epoch 1999.0, i.e. when the star was back to low brightness, observed with the HST. Superimposed is the synthetic model C scaled to a luminosity of $\log L/L_{\odot} = 4.58$ (dotted line). While the uppermost panel shows absolute fluxes, in the lower three panels the observed fluxes were divided by the model continuum.
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In the text

$\begin{figure} \par\includegraphics[width=8cm,clip]{h4416f4.ps} \end{figure}$ Figure 4: HST observation (grey thick line) of epoch 1996.0 compared with three models (black thin lines) of different hydrogen abundances (mass fractions $X_{\rm H} = 0.40$ , 0.20 and 0.05, respectively). Along the He II Pickering line series, every other member is blended with a H Balmer line and therefore stronger, compared to the monotonic decrement expected from hydrogen-free atmosphere. The model with $X_{\rm H} = 0.20$ roughly reproduces the observed up-and-down, while hydrogen mass fractions as high as 40% or as low as 5% can be ruled out.
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In the text

$\begin{figure} \par\includegraphics[width=7cm,clip]{h4416f5.ps} \end{figure}$ Figure 5: HST observation (grey thick line) of epoch 2000.1 compared with three models (black thin lines) of different carbon abundances (logarithm of the mass fraction $\log(X_{\rm C}) = -4.5$ , -4.0 and -3.5, respectively. Narrow interstellar absorptions are superimposed on a weak stellar P Cygni profile. The model with $\log X_{\rm C} = -4.0$ predicts an emission of about the observed strength, but a much stronger absorption component than observed.
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In the text

$\begin{figure} \par\includegraphics[width=8.8cm,clip]{h4416f6.ps} \end{figure}$ Figure 6: Luminosity and mass-loss rate of N 66 as function of time.
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In the text

$\begin{figure} \par\includegraphics[width=8.8cm,clip]{h4416f7.ps} \end{figure}$ Figure 7: Correlation between mass-loss rate and luminosity of N66 for the different epochs. The grey thick stripe indicates a slope of 0.87.
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In the text

$\begin{figure} \par\includegraphics[width=8.8cm,clip]{h4416f8.ps} \end{figure}$ Figure 8: Hertzsprung-Russell diagram with the path of N 66 between quiet state and outburst. For comparison, some other hydrogen-deficient (but [WC] type) central stars of well-determined luminosities based on known distances are given (full symbols). LMC-SMP 61 (d = 50 kpc from LMC membership) has been analyzed recently by Gräfener et al. (2003). The results for BD +30 $^\circ$ 3639 (d = 1.2 kpc from the secular expansion of the nebula) by Leuenhagen et al. (1996) and Crowther et al. (2003) are represented by a filled square and dot, respectively. The distances of He 2-113 and CPD -56 $^\circ$ 8032 (open symbols, analyzed also by Leuenhagen et al. 1996) are from less reliable methods. Post-AGB evolutionary tracks (from Blöcker 1993, 1995; Paczynski 1970) are shown for rough comparison and reveal that [WC] type central stars have the same typical mass around 0.6 $M_\odot$ as hydrogen-rich central stars. The outbursting [WN] star N 66,however, is much more luminous. On the other hand, N 66 is less luminous than any of the Galactic massive early-type WN stars (open symbols) analyzed by Hamann & Koesterke (1998a). Standard evolutionary tracks for massive stars (as given for an initial mass of 25 $M_\odot$ , from Schaller et al. 1992) do not lead to WN stars at such luminosities, as they end in a supernova explosion as red supergiants.
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In the text

$\begin{figure} \par\includegraphics[width=18cm,clip]{h4416f1.ps} \end{figure}$	Figure 1: The UV spectrum of N 66 at different epochs, showing the dramatic rise and fall of the stellar flux. Note that nebular emission strongly contaminates the IUE observations, but is effectively suppressed by the small apertures of the HST spectrographs.
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$\begin{figure} \par\includegraphics[width=18cm,clip]{h4416f3.ps} \end{figure}$	Figure 3: Spectrum of N 66 (continuous line) at epoch 1999.0, i.e. when the star was back to low brightness, observed with the HST. Superimposed is the synthetic model C scaled to a luminosity of $\log L/L_{\odot} = 4.58$ (dotted line). While the uppermost panel shows absolute fluxes, in the lower three panels the observed fluxes were divided by the model continuum.
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$\begin{figure} \par\includegraphics[width=8.8cm,clip]{h4416f6.ps} \end{figure}$	Figure 6: Luminosity and mass-loss rate of N 66 as function of time.
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$\begin{figure} \par\includegraphics[width=8.8cm,clip]{h4416f7.ps} \end{figure}$	Figure 7: Correlation between mass-loss rate and luminosity of N66 for the different epochs. The grey thick stripe indicates a slope of 0.87.
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