Evolution of the FWHM of the Hα emission line following the eruption of M31N 2008-12a. Black points indicate WHT and LT spectra from the 2014 eruption, grey points are from the 2013 eruption (TBW14), and the red point is from the HET spectra in 2012 (DWB14). The solid black line shows a simple linear least-squares fit to the 2012, 2013, and 2014 data (gradient = − 280 ± 50 km s^-1 day^-1), the grey dashed line a power-law of index −1/3, the red dot-dashed line a power-law of index −1/2, and the black dotted line the best fit power-law with index −0.12 ± 0.05, see the text for details.