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An O star whose spectrum displays strong N III 4634-4640-4642 emission and strong
He II 4686 emission. The N III lines are always much stronger than C III 4647-4650-4651
when the latter are present. Historically, Of stars were considered to belong to the
peculiar category, hence the f notation (see below). In his thesis work, Walborn removed
them from that category and established them as the normal O-type supergiants. He
also used the notation ((f)), (f), and f to describe the progression from strong He
II 4686 absorption, through weakened/absent, to emission, respectively, correlated
with increasing N III emission strength, subsequently showing that it is a luminosity
sequence -- the first such for stars earlier than O9.
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