Series of consecutive spectra around the Si III absorption line at 4553 Å, of the Cephei star 12 Lac (<V> = 5.25). The AURELIE grating N° 2 (R = 15 000) has been used. The obervations spread over the whole pulsating period of 12 Lac (4 h 38 min). There is typically 6 min 24 s between two spectra. Note the presence of a doubling line phenomenon around phase 1.317.