Far ultraviolet photometric observations (in 5 bands between λ = 1550 Å and 3300 Å) are presented for ten O stars which are also the exciting stars of diffuse nebulae. Since the number of photons shortwards of the Lyman limit is known for these stars, sufficient information on the total flux is available to determine the effective temperature without making any assumptions concerning a model atmosphere. Conversely, the distribution of flux with wavelength can be used to determine the applicability of a given model. A detailed discussion of this is presented.

A comparison of these ‘normal’ O stars and stars with O type spectra which excite planetary nebulae is given. It is concluded that the atmospheres of these two types of O stars are very similar; probably they have the same effective gravity. Further, a discussion of the Zanstra He II temperature is presented.