Variation of the scaling function for vsini with temperature and time between observations. Plot a) shows how the β parameter in Eq. (3) varies with T_eff. The solid line shows results using T_eff from a detailed spectral analysis; the dashed line shows the results using the “template” temperature (see Sect. 4.4). Labels indicate the sample size per bin. Plot b) shows the CDF of the normalised distributions of vsini uncertainty for short and long term repeats. Also shown (as small diamonds) is a Student’s t-distribution with ν = 2.