  Intrigued by the extended red-giant clump (RC) stretching across the
colour-magnitude diagram of the stars in a 50x50 pc^2 region of the Large
Magellanic Cloud (LMC) containing the clusters NGC 1938 and NGC 1939, we have
studied the stellar populations to learn about the properties of the
interstellar medium (ISM) in this area. The extended RC is caused by a large
and uneven amount of extinction across the field. Its slope reveals anomalous
extinction properties, with Av/E(B-V)=4.3, indicating the presence of an
additional grey component in the optical contributing about 30% of the total
extinction in the field and requiring big grains to be about twice as abundant
as in the diffuse ISM. This appears to be consistent with the amount of big
grains injected into the surrounding ISM by the about 70 SNII explosions
estimated to have occurred during the lifetime of the ~120 Myr old NGC 1938.
Although this cluster appears today relatively small and would be hard to
detect beyond the distance of M 31, with an estimated initial mass of ~4800
Msun NGC 1938 appears to have seriously altered the extinction properties in a
wide area. This has important implications for the interpretation of
luminosities and masses of star-forming galaxies, both nearby and in the early
universe.
