Evolution of the Hα line profile during the fireball phase of V339 Del (grey lines). Each profile was fitted with three Gaussian functions (dotted curves) to isolate its main components (Sect. 3.3). Numbers on the right side mark the shift of the profile in the given units for a better visualization. Dates are marked at the left side. Fluxes are in 10^-11 erg cm^-2 s^-1 Å^-1.