The long-term behaviour of the pulsation periods and amplitudes of the beta Cephei star 16 (EN) Lacertae

The blue-light observations of this pulsating and eclipsing variable (BCEPEA) cover an interval from 1950 to 1992. The out-of-eclipse (that is, pulsa tional) variation contains three sinusoidal terms with variable amplitudes. In our 1996 paper (JP96), based on the 1950-1983 data, we showed that for the first two terms, having frequencies equal to about 5.9112 and 5.8551 d( -1), the amplitudes varied with time-scales of the order of 50 yr. The rema ining term formed a doublet with constant amplitudes and frequencies close to 5.5033 d(-1), resulting in a beat-period of 1.85 yr. In the present pape r we confirm this picture using additional data, derive improved values of the time-scales and refine the parameters of the doublet. In addition, we d etermine the year-to-year variation of the epochs of maximum light of the f irst two terms. This allows us to account for (1) the ground-based blue-fil ter observations that were not extensive enough to be used in the analysis, and (2) the Hp magnitudes from the Hipparcos mission. In particular, we de monstrate that the period of 0.171 203 d, given in the Hipparcos Variabilit y Annex, is spurious.