The OB associations LH 101 and LH 104 in the HII region N158 of the LMC

We present photometric and spectroscopic ob- servations of stars in the Large Magellanic Cloud OB as- sociations LH 101 and LH 104, located in the HII region N158, which we have also imaged. From our observations we have constructed upper H R diagrams for these OB associations, which we nd to consist mainly of three pop- ulations, one of 2 6 Myr for the stars inside the northern bubble (LH 104), and two populations in the southern HII region (LH 101), one of 2 Myr and the other one aged 3 6 Myr. We have obtained for LH 101 a normal IMF, with a slope of = 1:29 0.20 whereas for LH 104 the IMF is flatter with a slope of = 1:05 0.12. These IMF slopes are consistent with that of other OB associa- tions in the LMC. Our observations reveal in the region of LH 101 the presence of both unevolved and evolved very massive stars, whose ionizing flux is in excess of that derived from our H images of the HII region. The north- west nebulosity in the region of LH 101 thus appears to be matter bound.Facultad de Ciencias Astronómicas y Geofísica