Photometry of bright stars in the Large Magellanic Cloud.

Photoelectric photometry in the UBV system down to a limiting visual magnitude of 14m9. is presented for 68 stars in two fields in the northern part of the Large Magellanic Cloud (LMC). The first field is located around the triple association NGC 1871/1869/1873, the second one in the surroundings of the stellar aggregate NGC 2034. Because of the small interstellar absorption in the LMC, it is possible to distingulsh a large fraction of the early-type supergiant stars which are probable LMC members, from main sequence or giant foreground stars by plotting the (U-B) vs. (B-V) two-colour diagram. About two-thirds of the stars selected in the two fields are probably physical members of the LMC. For the probable LMC member stars, photometric spectral types and luminosity classes are derived from the two-colour and the colour-magnitude diagrams, with an adopted true distance modulus (Vo-My) = 18m6.. Colour excesses are obtained for 32 LMC members and 14 galactic foreground stars, by comparing the observed colours with the intrinsic colours of corresponding unreddened galactic stars. The mean colour excess E(B V) found is ()` 24+O' 15 for 6 LMC emission-line stars, as compared to O ! O7+O' O7 for 26 LMC supergiant stars without known emission features in their spectra, and to OmO.3 +OmO.3 for 14 galactic foreground stars. Among the six LMC clusters and associations included in this study, the triple associaflon NGC 1871/ 1869/1873 is particularly conspicuous. It contains at least 14 supergiants with bolometric magnitudes exceeding -7m()., including the WoIf-Rayet star HI) 34632 (WN4+OB), the peculiar Fe H emission-line star HI) 34664 and the variable red supergiant HV 5657. The ages of the supergiants in this triple association are shown to be different, ranging between 8 x 106 and 13 x 106 years. The youngest and brightest supergiants are located in NGC 1871 and NGC 1873, outside the central association of NGC 1869. NGC 1869 and NGC 1871 are embedded in a large H II region. Key words: Magellanic clouds - stellar photometry - open clusters - supergiants - interstellar matter