AR Lacertae: Hα chromospheric emission and radial velocities from long-term optical spectroscopy

We present here the results of spectroscopic studies in the Hα region of the active eclipsing binary AR Lac (G2IV+K0IV) obtained at Catania Astrophysical Observatory in 1989, 1990, 1994, 1996 and 1997. Accurate measurements of radial velocities by using the cross-correlation method have been made and a new orbital solution has been derived. The equivalent width of the Hα emission core has been measured by subtraction of spectra synthesised from standard non-active stars of spectral types similar to those of the two components. We find chromospheric emission from both components in most of the spectra. No rotational modulation of the Hα emission is apparent. The absorption excess observed near and during primary minima in 1997 is interpreted as the effect of a prominence-like structure anchored between the leading and the trailing hemisphere of the cool component. This interpretation is fully compatible with the radial velocities of the Hα emission peaks in the subtracted spectra. The observation of a Hα flare in 1989 on the cool star is also reported and an estimate of the size of the magnetic structure involved in the event is proposed.