Geneva data

The block of 124 data for HD 220392 covers an interval of 464 days (Table 1). We used the frequency step of 5.8 10^-5 cpd ( $\simeq 1/20{\rm T}$) with the PERIOD98 software (Sperl, 1998). After the Fourier analysis, the frequencies, amplitudes and phases were improved by a least squares fit that gave a main frequency around 4.679 cpd, the same one as previously reported by Lampens (1992). The standard deviation dropped by more than 28% after prewhitening for this frequency. Since the theoretically expected noise level of 0.006 mag for a bright constant star observed in the Geneva Photometric System (Rufener, 1988) was not yet reached, a search for a second frequency in the prewhitened data was performed, revealing either 5.520 or 6.520 cpd. The second highest amplitude was found for a two-frequency fit with 5.520 cpd: results of both simultaneous fits are presented in Table 2(a). After prewhitening for the frequencies 4.679 and 5.520 cpd, the residual standard deviation falls to 0.0085 mag, still larger than expected. However, from an inspection of the (night-to-night) plots of the phase diagrams, we have noticed that the 7 data points on JD 2448518 have a level that is about 0.01 mag off compared to the rest of the data. This accounts for an extra 0.001 mag residual dispersion.

Table 2: Results of a two-frequency fit (a) for the Geneva data (b) for all 396 data of HD 220392 (program PERIOD98)

Dataset	Frequency	Semi-ampl.	Residual $\sigma$	Reduction
	cpd	mag	mag	%
(a)	f1, 4.679	0.0137	0.01070	28.7
	(f2, 6.520)	0.0096	0.00845	15.1
	f1, 4.679	0.0128	0.01070	28.7
	f2, 5.520	0.0099	0.00845	15.1
(b)	f1, 4.67437	0.0155	0.00911	40.8
	f2, 6.52260	0.0097	0.00644	17.3
	f1, 4.67439	0.0139	0.00915	40.6
	f2, 5.52234	0.0110	0.00614	19.5