Probing interstellar extinction near the 30 Doradus nebula with red giant stars

We have studied the interstellar extinction in a field of similar to 3 arcmin x 3 arcmin located about 6 arcmin southwest of 30 Doradus in the Large Magellanic Cloud (LMC). Hubble Space Telescope observations in the U, B, V, I and H alpha bands reveal patchy extinction in this field. The colour-magnitude diagram (CMD) shows an elongated stellar sequence, almost parallel to the main sequence (MS), which is in reality made up of stars of the red giant clump (RC) spread across the CMD by the uneven levels of extinction in this region. Since these objects are all at the same distance from us and share very similar physical properties, we can derive quantitatively both the extinction law in the range 3000-8000 angstrom and the absolute extinction towards about 100 objects, setting statistically significant constraints on the dust grains properties in this area. We find an extinction curve considerably flatter than the standard Galactic one and than those obtained before for the LMC. The derived value of R-V = 5.6 +/- 0.3 implies that in this region larger grains dominate. Upper MS stars span a narrower range of E(B - V) values than RC objects, at variance with what has been found elsewhere in the LMC.