Phase-averaged luminosity and centroid energy of the absorption lines. The horizontal cyan bands are the energy range of the most negative phase shift of the main peak measured from the BeppoSAX data. Uncertainties on fitted parameters are at 90% c.l. Upper left: X-ray luminosity (see Table 1). Upper right: centroid energy of the fundamental. Lower left: centroid energy of the first harmonic. Lower right: centroid energy of the second harmonic.