SIMBAD references

The ages of the young associations located at the border of the supergiant shell LMC4 are essential parameters to understand the formation and the evolution of that supergiant shell. We present spectra in the UV and the optical wavelength range of eleven stars in NGC 1948 in order to establish the location of the top of the main sequence more accurately than possible from photometry alone. From the present data we derive an age for NGC 1948 of 5-10Myr, a much tighter range than derived earlier by Vallenari et al. (1993A&A...268..137V), but located at the lower border of the age range given by those authors. Two stars seem to be as old as 40Myr, but from their location at the border of the association they may be non-members. The given range of ages corresponds to a real spread in the ages of the sample stars and not to an error estimate of a precisely fixed age.