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Emission lines from H II regions and planetary nebulae are contaminated by shock waves produced by stellar winds and supernova remnants. This effect is studied and it is found that it could be responsible for: (a) the difference between the O/H abundances derived from the O nebular lines coupled with H II region models and those derived from the observed I (4363)// (5007) ratio, (b) the large t 2 values observed in the Orion nebula and M 8, (c) the difference between the stellar and the H II region O/H abundances in the solar neighborhood, (d) the large [S n]/Ha ratios observed in H II galaxies, and (e) the low O/H ratios and the N/O versus O/H anticorrelation found in Type I PN and shell nebulae around Population I stars.

Photoelectric spectrophotometry in the 3400 A-7400 A range is presented for five H II regions in the spiral galaxy NGC 2403. Steep gradients in the electron temperature, N/H, and O/H were found. By adopting the Y versus O/H relation our observations also imply gradients in He°/H + . The maximum temperature of the most massive stars in giant H II regions across the disk of NGC 2403 is 35,000 K; this result implies that the mass of the most massive stars is very similar in each HII region and that it is independent of the gradient in heavy element abundances. By comparing the derived chemical abundances with models of galactic chemical evolution it is found that infall into the galactic disk is not important for NGC 2403.

The central star of NGC 2346 is a well known binary with an A-type primary and a hot companion (Méndez and Niemela 1981). The star went through a series of periodic light variations which ceased in 1986 and were interpreted as an eclipse of a dust cloud passing in front of the binary system (e.g. Méndez et al. 1982, Costero et al. 1986). Recently, light variations reappeared with shallower minima compared to the previous eclipse (e.g. Kohoutek et al. 1992).

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