Astron. Astrophys. 332, 605-609 (1998)

2. Periodicities

A period search was carried out using Fourier analysis on the differential A9007 minus B9007 LTPV data in the frequency range 0.0-0.2 cd^-1. The spectral window (Fig. 2, top) is dominated by a strong peak at 0.00276 cycles per day (cd^-1), and a further 1 cycle per year alias at 0.005525 cd^-1 due to the annual cycle of our observations. The amplitude spectrum (Fig. 2, bottom) shows the strongest peak at cd^-1 with a weaker peak for the 1 cycle per year alias 0.00405 cd^-1. A least-squares sine fit with this frequency reduces the standard deviation by little more than 0:m01 in all bands, leading to residuals that are still a factor of four larger than expected. A least-squares sine fit in each band leads to an average cd^-1 ( d, for the LTPV data).

Fig. 2. Spectral window (top) and amplitude spectrum (bottom) of HD 163868 (y) in the frequency range 0.0-0.05 cd-1

Besides the light curve published by Woodward (1975), the literature contains only one strongly deviant visual magnitude of HD 163868, that is, Kozok (1985) who reports on HJD 2444025. Though we should not attach too much importance to this number in absolute terms, it is notwithstanding the brightest V -band measure ever reported, so we feel confident when identifying this bright phase with the maximum of a brightening event as the one seen by Woodward (1975). With our tentative period of the order of 830 days, we can construct a cycle-count scheme in which the maximum assigned to the measurement of Kozok (1985) occurs two cycles after the very first maximum. Then, four cycles would elapse till the supposed maximum around JD 2447600-7700 (no data available), after which brightness peaks around JD 2448500 and 2449200 each follow at one cycle interval. This scheme then leads to a preliminary linear ephemeris

We stress that this "period" should not be regarded as rigid in the true sense of the word, but that we rather deal with semi-periodic behaviour. As such, the mean errors quoted with the ephemeris only represent the uncertainties in the mathematical solution.

© European Southern Observatory (ESO) 1998

Online publication: March 23, 1998
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