The extinction curve in the visible and the value of Rv
  This article discusses the interstellar extinction curve in the visible and the value of Rv. It is concluded that the visible extinction curve is likely to be linear in the visible, and that indirect estimates of Rv from tentative determinations of Av, infrared, or UV observations are questionable. There is currently no evidence of any variation of Rv with direction. If Rv is close to 3, as it has been inferred from mid-infrared data, starlight in the visible is extinguished by a factor F/F_0=(2.5exp{-2micron/lambda})^{E(B-V)} in the visible. But if the visible wavelength range alone is considered, 4 appears as its most natural and probable value, and F/F_0= exp{-2E(B-V)/lambda}. 
Frederic Zagury
astro-ph.GA