The colour-lightcurve shape relation of type Ia supernovae and the reddening law

Aims. We present a study of the time sequence of optical colours of a large sample of nearby type Ia supernovae (SNe Ia). We study the dependence of the colour time-evolution on the lightcurve shape, parametrized by the stretch factor. Methods. We determine the spectral template that minimizes the dispersion in the colours of SNe Ia, measured by UBVRI photometry of nearby supernovae. Results. A clear colour dependence upon lightcurve shape is found, such that the narrower lightcurves are redder up to about one month past lightcurve maximum. After correcting for lightcurve-shape differences in intrinsic colour, we derive an average reddening law, that is well described by a Cardelli, Clayton & Mathis law with R-V = 1.75 +/- 0.27 for 80 type Ia supernovae with E(B - V) = 0.7 mag. A subset of 69 SNe that have modest reddening, E(B - V) < 0.25 mag, gives a significantly smaller value, R-V similar to 1, which suggests that the observed reddening of type Ia supernovae may have a more complex origin, perhaps associated with other processes besides extinction by interstellar dust inside the host galaxy.