Spectrum of Hβ (left panels) and of Mgiiλ2800 (right panels) for the B1⁺⁺ source PG 1201+436. Color coding is the same as for Fig. 2 save for the blue line that traces a blueshifted peak visible in both Hβ and Mgiiλ2800 (Sects. 4 and 5.3). Horizontal scale is rest frame wavelength or radial velocity shift from rest frame. The vertical scale is specific flux in units of 10^-15 erg s^-1 cm^-2 Å^-1.