7 Summary

We combined new observations of the LH 101 OB association with the previously published data of LH 104, in order to construct upper H-R diagrams of the stellar population of the two regions. We find the following:

- LH 101 consists of at least two age groups, one with an age of $\leq$ 2Myr mainly composed of stars inside the nebulosity, and the other of stars outside the nebulosity with an age of 3-6 Myr. LH 104 appears to be composed only of one group of 2-6 Myr.

- The IMF slope of LH 101 with $\Gamma$ $\leq -1.29 \pm 0.20$ seems to be well represented by a Salpeter slope, whereas for LH 104 the IMF slope is flatter with $\Gamma\sim -1.05$ $\pm$ 0.12, which probably is the result of the presence of the binary systems containing WR stars.

- LH 101 contains within the nebulosity to the S-E several reddened unevolved early O type stars, and in the N-W portion unreddened evolved massive stars outside the nebulosity, suggesting that even in LH 101 a propagating star formation is present, probably on a scale of the order of 50 pc.

- The ionizing flux provided by the early O stars present in N158C is in excess of that derived from the H$\beta$ luminosity of our narrow band image. Therefore, the H$\beta$ or H$\alpha$ luminosity may not be a reliable indicator of stellar content in sites of massive star formation.