1 Introduction

The Large Magellanic Cloud (LMC) is rich in HII regions and young OB associations and, because of its known and relatively small distance, it is well suited to CCD observations and is an ideal laboratory in which to study the formation and evolution of massive stars. We therefore embarked on detailed studies of such regions. In this paper we will deal with the OB associations LH 101 and LH 104 (Lucke & Hodge 1970), which were also included in the photographic photometry of OB associations in the Magellanic Clouds performed by Westerlund (1961). These associations are inside the isolated elongated giant HII region N 158 (Henize 1956). This HII region is located 20$^\prime$ south of the nucleus of 30 Dor and stretches over 11$^\prime$ (equivalent to $\sim$ 160 pc) in the NE-SW direction. Its northern part is composed of a superbubble around the OB association LH 104, while the southern part containing the OB association LH 101 is a diffuse HII region which is characterized by three bright zones. The whole region belongs to a large complex of ionized gas in the northern part of a giant molecular cloud bordering the two young neighbour associations in the nebulae N159 and N160 (Israel et al. 1996).

Lortet $\&$ Testor (1988) have studied the morphology of this region, and described conspicuous sequential star formation from the stellar association LH 104 toward the most excited parts of N158C. The sequential star formation in N158 is strengthened by recent studies showing that LH 104 is dominated by B stars (Schild $\&$ Testor 1992) and LH 101 by unevolved and evolved O-type massive stars (Testor $\&$ Niemela 1996), suggesting that this region is an active site of very massive star formation.

Integrated H$\alpha$ and H$\beta$ fluxes of N 158 through diaphragms have been measured by Kennicut & Hodge (1986) and Caplan & Deharveng (1985). Jones et al. (1986) detected two infrared sources in LH 101: NIR 1 and NIR 2. NIR 1 has colours typical of a reddened O star of luminosity $\sim 2 \ 10^{6}\,L_{\odot}$

The region of N 158 is also the third richest area of the LMC for WR stars. Nevertheless, a detailed examination of the stellar content of LH 101/104 is lacking, perhaps because the bright nebulosity hinders accurate stellar photometry. In this paper we present:

- in Sect. 2 the data base, consisting of new observational material of stars in LH 101, namely UBV CCD photometry and spectra, and previous imagery and data. The data reduction closely follows the study of LH 90 by Testor $\&$ Schild (1993).

- in Sect. 3 a description of the spectra of peculiar stars, and the H-R diagrams of the two associations with a description of their age structure.

- in Sects. 4 and 5 the determination of the initial mass function (IMF) for both OB associations, and for the HII region of LH 101 a comparison between its H$\alpha$ luminosity and the Lyman ionizing flux.