MODE-IDENTIFICATION OF THE BETA-CEPHEI STAR BW VULPECULAE

We analyse the pulsational behaviour of the large amplitude beta Cephe i star BW Vulpeculae from high-resolution spectra. Our observations sh ow very clear line-doublings at the time of maximum outward accelerati on. These line-doublings appear and disappear on a time scale of about 30 minutes and lead to discontinuities in the radial-velocity curve. From our observations and photometric data we identify the pulsation m ode of BW Vulpeculae by applying respectively the moment method (see e .g. Aerts et al. 1992) and the method of photometric amplitudes (see e .g. Heynderickx et al. 1994). Although a strong non-linear behaviour i nvolving shock waves is suspected in the case of BW Vulpeculae (Crowe & Gillet 1989), both mode-identification techniques, which are develop ed for linear motions only, point towards a radial pulsation. Combinin g our result with the one recently found by Moskalik & Buchler (1994) then leads to the conclusion that BW Vulpeculae pulsates in the fundam ental radial mode.