Anomalous extinction behaviour towards the Type Ia SN 2003cg

We present optical and near-infrared photometry and spectroscopy of the Type Ia SN 2003cg, which exploded in the nearby galaxy NGC 3169. The observations cover a period between -8.5 and +414d post-maximum. SN 2003cg is a normal but highly reddened Type Ia event. Its B magnitude at maximum B_max = 15.94 +/- 0.04 and Δm₁₅(B)_obs = 1.12 +/- 0.04 [Δm₁₅(B)_intrinsic = 1.25 +/- 0.05]. Allowing R_V to become a free parameter within the Cardelli et al. extinction law, simultaneous matches to a range of colour curves of normal SNe Ia yielded E(B - V) = 1.33 +/- 0.11, and R_V = 1.80 +/- 0.19. While the value obtained for R_V is small, such values have been invoked in the past, and may imply a grain size which is small compared with the average value for the local interstellar medium.