Determination of RV towards galactic O stars

We present new measurements of the interstellar reddening parameter R-V = A(V)/E(B-V) towards 35 O stars. The results are combined with measurements from the literature for 60 stars to study the behaviour of R-V with heliocentric distance. R-V is the single basic parameter which characterizes the interstellar extinction from the near-infrared to the far-UV spectral region. The absolute extinction A(V), from which R-V is derived, is best determined by optical and near-infrared photometry (Cardelli et al. 1989). We consider important the derivation of R-V with the same technique in the direction of as many as possible galactic O stars.