## 2. Observations and period analysisAdditional photometry was carried out in December 1993, again in
the
The period search is performed by looking for the best fit of a mathematical model to the observational data. The fitting function is a cosine wave and its first harmonic: where The use of this period-searching algorithm has two advantages. Firstly, Eq. (1) is known to be a very good approximation for most well-studied Ap stars; higher-order harmonics, if at all present, are of small amplitudes (see, e.g., Mathys & Manfroid 1985, Manfroid & Renson 1994). Secondly, the zero-point differences between the Danish and SAT photometric systems are automatically adjusted by the procedure for each trial period. This is of little concern for large data sets with homogeneous phase coverage, where the zero-point shifts can be calculated from simple averages. With only a small observational material at our disposal, it becomes a big asset since the zero points of peculiar stars cannot be accurately estimated from regular standard-star transformations. Because of the 12-year gap separating both sets of observations, an
unambiguous determination of the period proves to be impossible, with
dozens of equally likely candidates spread over a rather broad
interval centered at 8:d6 (see Fig. 1 for the periodogram
relative to the
A few additional measurements of the star have been secured in
September 1993 at the ESO 1 m telescope in a special narrow-band
filter having the same central wavelength as Strömgren
Lifting the remaining ambiguity was made possible by the analysis of the variations of the mean magnetic field modulus (Mathys et al. 1996). 31 magnetic mesurements were secured over two years. The resulting periodogram obtained with the same method as for the photometric data is shown on Fig. 3.
By combining the photometric and magnetic results, the most likely period appears to be . The next candidate is which, although less probable, cannot be totally excluded. The
HD 208217 is characterized by strongly anharmonic light
curves. The amplitude of the first harmonic is practically equal to
that of the fundamental wave in each band. A search for higher-order
harmonics hints at a marginally significant second-order wave in the
Although their ratio stays more or less constant, the amplitudes
and change dramatically
with wavelength, with a sharp maximum in the The phase diagram of the mean magnetic field modulus, with a two-wave fit superimposed, is shown on Fig. 5. The coefficients of the fit are given in the last line of Table 1. While the presence of a first harmonic in the magnetic variations is now well established, the observational errors do not allow to conclude on possible higher-order terms.
© European Southern Observatory (ESO) 1997 Online publication: June 30, 1998 |