H II regions which exhibit surprisingly strong nebular He II λ4686 emission, with I(λ4686)/I (Hβ) as high as 0.3 in some cases. The He II emission in-these objects is associated with a rather diverse set of stars, with WO stars, very early WN stars, and luminous X-ray binaries involved, and in one case what appears to be a normal O-type star. Extension of a thermal X-ray spectrum to lower energies appears to satisfactorily explain the He II emission associated with the X-ray binaries. For the remaining objects, stellar effective temperatures of at least 60,000 K are required to account for the observed He II emission, if photoionization by a hot stellar continuum is responsible. The existence of stars hot enough to produce such emission presents an interesting problem. for stellar atmospheric models and stellar evolution and may provide an explanation for the He II λ 4686 emission commonly observed in metal-poor extragalactic H II regions.
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Garnett et al. (1991) studied this question.