BW Vul is remarkable for exciting an extremely strong radial pulsation mode .
This instability grows in its outer envelope and forms visible shock features in the continuum flux and spectral line profiles at two phases separated by 0.8 cycles .
Material propelled upwards energetically in the atmosphere from the shock returns to the lower photosphere where it creates a second shock just before the start of the next cycle .
We have obtained three nights of echelle data for this star over about five pulsation cycles ( P = 0.201 days ) in order to evaluate the effects of atmospheric shocks on a number of important red lines in the spectrum .
These lines include He I \lambda 5875 and \lambda 6678 , C II \lambda \lambda 6578-83 doublet , and other moderate ( e.g. , Si II \lambda 6371 ) and high excitation ( Si III \lambda 5737 ) lines .
We have added to these data 37 archival IUE/SWP echelle spectra obtained in 1994 .
We have investigated the equivalent widths and shapes of the optical lines for evidence of inter alia lags and have compared our results to the IUE fluxes extracted from the far-UV continuum , He II \lambda 1640 , and several resonance lines .

A comparison of He I \lambda 5875 and \lambda 6678 line profiles during the peak of the infall activity suggests that differences in the development of the blue wing at this time are due to heating and a short-lived formation of an optically thin layer above the region compressed by the infall .
This discovery and the well-known decreases in equivalent widths of the C II doublet at the two shock phases leads us to suggest that shock heating flattens the atmospheric temperature gradient , whether it is the infall shock preferentially heating of the upper atmospheric layers from infall , or the pulsational wave shock , which takes on an isothermal character as it emerges into the more tenuous upper photosphere .

Except for evidence of wind in the far blue wings of the UV resonance lines , we find no evidence for a shock delay arriving at different regions of line formation of the photosphere ( i.e. , a “ Van Hoof effect ’ ) .
Phase lags attributed by some former observers may be false indicators arising from varying degrees of desaturation of multiple lines , such as for the red He I lines .
In addition , an apparent lag in the equivalent width curve of lines arising from less excited atomic levels could instead be caused by post-shock cooling , followed by a rebound shock , as suggested by subtle variations in the photospheric \lambda 1640 and UV continuum flux curves .