3 Frequency analysis

The frequency analysis on the three data sets was performed using the PDM method (Stellingwerf 1978) and the CLEAN method (Roberts et al. 1987). We tested frequencies from 0 to 3 cycles per day (c/d) with a frequency step of 0.0001 c/d and we searched for multiple periods by prewhitening.

Figure 1: Phase diagram of the U-magnitude data.

 

 

Table 2: Standard deviation of the Geneva data in the different filters and of the residuals after fitting with a sine for the frequency 0.6374 c/d.

Filter	Standard deviation
	Data	Residuals
U	0.0151	0.0091
B1	0.0120	0.0072
B	0.0106	0.0065
B2	0.0111	0.0073
V1	0.0090	0.0058
V	0.0091	0.0055
G	0.0096	0.0059

The frequency of 0.6374 c/d is found in the Hipparcos photometry. The same frequency is obtained from the Geneva data in the 7 filters. In Table 2 the standard deviation of the data in the different filters is given as well as the one of the residuals after fitting with a sine. A phase diagram for the U-filter is shown in Fig.1. This frequency reduces the standard deviation in the U filter by 40%. An additional frequency cannot be found either in the Hipparcos data or in the Geneva data.

From the doublet SiII centered at $\lambda\lambda\,4128, 4130$ Å we computed the first moment and performed a frequency analysis. We again obtained the frequency 0.6374 c/d. A sine fit to the first moment for this frequency explains 67% of the standard deviation and a slightly better fit leading to a reduction in standard deviation of 69% is obtained with its first harmonic included (see upper panel of Fig.2). No other frequency can be found in the data and it is clear that this star is monoperiodic.

We note that, if the variability is due to pulsation, such a long period is attributed to a g-mode. We also note that the observed period may be the rotation period of the star.