SIMBAD references

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., 1989ApJ...345..245C). We consider important the derivation of R_V with the same technique in the direction of as many as possible galactic O stars.