SIMBAD references

A full-amplitude pulsating model for BL Her is generated, and the Hα formation in the shocked extended atmosphere is studied in the non-LTE approximation. Theoretical profiles represent qualitatively well the observed Hα variation discovered recently by Gillet et al. (1994). Like the RR Lyr model, the BL Her model developed a very extended atmosphere, periodically affected by a series of weak and strong shock waves. Three of them, with amplitudes below 40km/s, merge near the surface one by one into one strong shock with the total amplitude {DELTA}U≃110km/s, which causes the observed line doubling. Just before this phase the shock heating provokes a short-living emission. It is shown that in BL Her the shock, strong enough to produce a prominent emission, forms deeper than in RR Lyr, but higher than in W Vir model. This can explain why the observed emission strength in BL Her is intermediate between that in W Vir and RR Lyr stars.