... M 33

Based on observations obtained at the 4.2 m WHT telescope operated on the island of La Palma by the Isaac Newton Group in the Spanish Observatorio del Roque de Los Muchachos of the Instituto de Astrofisica de Canarias.

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... 0.15

Gurzadyan (1988) distinguishes between low and high excitation objects. We prefer to use the term "medium'' instead of low because when the He  II $\lambda$4686 line is measured the excitation cannot be really be regarded as low. We think the specification "low'' is more appropriate for PNe where the line is not detectable.

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