Spectroscopy of the roAp star   Cir - II. The bisector and equivalent width of the H  line

We present bisector measurements of the H α line of the rapidly oscillating Ap (roAp) star α Cir, obtained from dual-site observations with medium-dispersion spectrographs. The velocity amplitude and phase of the principal pulsation mode vary significantly, depending on the height in the H α line, including a phase reversal between the core and the wings of the line. This supports the theory, suggested in Paper I, of a radial pulsational node in the atmosphere of the star. Blending with metal lines partially affects the H α bisector results, but probably not enough to explain the phase reversal. We have also detected changes in the equivalent width of the line during the pulsation, and measured the oscillatory signal as a function of wavelength across the Hα region.