Measurement of the EW_min for the Mg II emission line of the OP 313 spectra analysed in this work. Top left: SDSS spectrum from 2006. Top middle: NOT spectrum from May 1. Top right: TNG spectrum from May 28 (first exposure). Centre left: TNG spectrum from May 28 (second exposure). Centre middle: NOT spectrum from June 18. Centre right: TNG spectrum from July 3 (first exposure). Bottom: TNG spectrum from July 3 (second exposure). Continuum regions are represented in blue and their vertical size corresponds to the 1σ noise value. The window of the emission line is highlighted in green and its size represents a 3σ noise value. The continuum interpolation is represented by the dotted-dashed line.