2003A&A...410..905P

We present new measurements of the interstellar reddening parameter R_V=A_V/E(B-V) towards 185 O stars, using J, H, K_s photometry from the 2MASS project. The results are combined with data from the literature of 95 stars where R_V has been derived with the same technique, 22 of which in common with our present sample from the 2MASS project catalogue. The average R_V from these 258 O stars is of 3.19±0.50. All objects whose R_V departs from this value by more than 2σ have been recognized. Ten objects have R_V higher than this value and two lower. It is found that anomalous R_V can scarcely be associated with anomalies in the general interstellar medium, e.g. with different behaviour in different spiral arms. They are clearly linked to local cloud effect. In the Cygnus region R_V values follow the behaviour of the general interstellar medium, while in the Carina arm, in spite of the relatively larger distance, local cloud effects prevail. An explanation for this is suggested. The relatively few stars of our sample whose Hipparcos parallaxes are reliable, are found to have distances systematically smaller than the distances derived by the spectroscopic parallaxes. We argue that this effect is consistent with the recently claimed discovery of grey extinction towards OB stars.