In this paper we presented Fourier analysis of lpv of He I, Fe II, Mg II and Si II lines in the Be star $\eta$ Cen (HD 127972) observed in six epochs at high resolution and S/N ratio from May 1996 to April 2001. The lpv were interpreted in terms of nonradial pulsations (NRPs). Time analysis was performed using Cleanest algorithm and showed the following frequencies with high order of significance: 0.61 c/d, 1.48 c/d, 3.81 c/d, 5.31 c/d, 9.24 c/d and 10.35 c/d. All signals except the first are interpreted as due to NRP modes. From phase variation diagrams we estimated $\ell$ parameters in the range 3-7. If the 10.35 c/d frequency is considered the first harmonic of 5.31 c/d, the corresponding azimuthal number of the mode is $\vert m \vert = 4\pm2$ . The observed multiperiodicity is in agreement with our latest works (Janot-Pacheco et al. 1999). The detected signal with 1.48 c/d is probably due to an NRP mode rather than to rotation, which is $\nu_{\rm r} \simeq$ 1.12 to 1.24 c/d (at most $\nu_{\rm r} =$ 1.39 c/d if the star were to be regarded as a critical rotator). The 3.81 c/d frequency could be interpreted as the multiple of 1.8 c/d ( $3.8 \sim 2 \times 1.9$ ), but 1.8 c/d was not found in LNA spectra (He I 6678 Å). It was detected in other wavelengths of ESO spectra. If this signal really existed, 3.81 c/d could be also interpreted as due to NRP. The only instance where we detected at the same time both a signal and its first harmonic was in the case of 5.31 c/d and 10.35 c/d signals, though the significance level of the latter frequency is less than 75%. In this case, the IPS method yields a tesseral mode $\ell = 5$ , $\vert m \vert = 4$ .

Table 6: CE parameters from fits of H $\alpha$ emission line profiles.
Epoch $\tau_{\rm o}$ R/R_* H/R_* $V_{\Omega}$ $V_{\rm r}$ $V_{\rm P}$ (R/R_*)_K

km s^-1

1996 0.9 3.5 3.1 260 40 210 2.2

1997 1.5 3.9 3.6 280 0 230 1.8

2000 0.7 7.3 5.5 260 0 150 4.3

2001 1.3 6.3 4.7 260 0 140 4.9

**Table 6:** CE parameters from fits of H $\alpha$ emission line profiles.
Epoch	$\tau_{\rm o}$	R/R_*	H/R_*	$V_{\Omega}$	$V_{\rm r}$	$V_{\rm P}$	(R/R_*)_K
				km s^-1
1996	0.9	3.5	3.1	260	40	210	2.2
1997	1.5	3.9	3.6	280	0	230	1.8
2000	0.7	7.3	5.5	260	0	150	4.3
2001	1.3	6.3	4.7	260	0	140	4.9

The photometric variations of HD 127972 observed by Hipparcos from 1990 to 1992 were also considered. They could be folded with a frequency $\nu_{\rm phot} =$ 1.55 c/d. According to the fundamental stellar parameters, this signal cannot be attributed to stellar rotation.

The fundamental stellar parameters were determined by taking into account the first order effects produced by fast stellar rotation on the BCD spectrophotometric quantities used to estimate them. According to these parameters, the star should be in the midst of its MS life span as expected by Fabregat & Torrejon (2000) for Be stars in open clusters. This also implies that, if the star becomes a fast rotator due to evolutionary effects as predicted by Maeder & Meynet (2001), the object would need to be a rapid rotator on the ZAMS.

The H $\alpha$ line profile was observed from May 1996 to April 2001. Its emission increased from 1996 to 2000 and then decreased slightly in 2001. The season-averaged H $\alpha$ line emission strengthening was correlated with an increase of the lpv wavelength extent in the He I $\lambda$ 6678 line (Figs. 11 and 14). The activity with high confidence level extended beyond the $V\sin i$ limit by 26% in 1997 and 45% in 2000. The signal $\nu =$ $0.6\pm0.1$ in the He I $\lambda$ 6678 line wings beyond the $V\sin i$ limit has an increasing level of significance from 1996 to 2000, which may indicate that the amount of matter gathered in the CE regions near the central star was growing during this period.

We used a simple model to study the characteristics of the season-averaged H $\alpha$ line emission profiles. The fit of model emission line profiles indicates that the H $\alpha$ variation from 1996 to 2000 is consistent with an increase of the amount of accumulated matter in the emitting region and with an enlargement of its extent. The 2000 to 2001 transition phase can be interpreted as due to shrinkage of the CE emitting region, where the amount of stored mass remained nearly unchanged.

7 Conclusions