1997A&A...318..269M

The profile, radial velocity and equivalent width of the interstellar lines of Na I (5890.0, 5895.9Å) and K I (7699.0Å) have been obtained from Echelle+CCD observations at resolving power λ/{DELTA}λ∼16,500 for 32 O and early B stars suffering from a reddening between E_B–V=0.06 and 1.57. The data have been used to search for and calibrate a relation between equivalent width and reddening. When the interstellar lines show a single and sharp component, useful relations to estimate reddening from equivalent widths have been derived. The relation for Na I is most sensitive in the range 0.0≤E_B–V≤0.4, and the one for K I takes over at higher reddening. Good quality equivalent width measurements allow E_B–V to be estimated with an accuracy of about 0.05 mag. For multi-component profiles of Na I and K I lines the estimate of reddening is more ambiguous with a general scatter of 0.15mag. Close blends of multiple components allow only an estimate of an upper limit to E_B–V.