PULSATION AND BINARITY IN BETA-CEPHEI STARS .2. 16-LACERTAE

Photometric and spectroscopic observations of the beta Cephei star 16 Lacertae obtained in 1983 and 1984 are presented and analysed. Using a ll the published maxima we have computed ephemerides for each of the t hree pulsational modes. No period variation is detected from 1950 to 1 983. In order to identify the pulsational modes corresponding to the t hree main periods of 16 Lac we have applied different methods based on photometric and radial velocity variations. All give the same results : the mode corresponding to the largest amplitude P-1 (0.(d)16917) is radial, P-2 (0(d).17079) is a nonradial mode of degree l = 2. For P-3 (0(d).18171) the results are more dubious, but P-3 is probably a l = 1 nonradial mode. We have also studied the interaction between pulsatio n and binarity in 16 Lac. We confirm the existence of a resonance phen omenon between the radial and nonradial modes: the ratios between the beat frequencies of the pulsational modes and the orbital frequency ar e close but significantly different from simple rational numbers. Acco rding to Kato (1974) that means that the nonradial modes might be exci ted or at least enhanced by tidal effects. They would have been select ed among all the possible modes because their ability to be resonant w ith the radial mode. The photometric amplitude of P-3 in both filters appears to be variable from night to night in correlation with the orb ital phase but despite its rapid and irregular fluctuations, the P-3 a mplitude stayed in constant average over 80 years. The present observa tions confirm the decrease of P-1 and P-2 amplitudes since 1950 but an analysis of older radial velocity data shows that the amplitude of th e P-1 mode went through a maximum around the middle of the century bef ore the observed decrease of the last thirty years.