COMMISSION 27 OF THE I.A.U.
                 INFORMATION BULLETIN ON VARIABLE STARS
                             Number 3282

                                                         Konkoly Observatory
                                                         Budapest
                                                         11 January 1989

                                                         HU ISSN 0374 - 0676


       The rotation period of the Ap star HD 220825 (Kappa Psc)

The Ap star HD 220825 was first monitored by Rakosch (1962), who determined a
rotation period of 0.5805 day and found the amplitude to vary less than 20
millimagnitudes (mmag) peak-to-peak in blue and yellow filters. A refinement
of the rotation period was undertaken by van Genderen (1971), who obtained
five-color Walraven photometry. He utilized a second comparison star, HD
221318, in addition to the one used by Rakosch (HD 220858), and found HD 220858
to be slightly variable. A new period of 0.5853 days was determined, based on
using HD 220858 as the sole comparison star.
   Spectroscopy by Schneider in 1988 revealed spectral line variation that did
not agree in phase with this period. For this reason, we decided to reobserve
HD 220825. Here, we present our first data obtained by Kreidl at Lowell
Observatory in September and October 1988.
   Photometry was obtained on seven nights with the Lowell 1.1-m telescope and
a narrowband (FWHM ~70 A) filter centered at 4060 A. As comparison stars, HD
221318 (which was utilized by van Genderen), and HD 221950 were used. Neither
comparison star showed any indications of variability; HD 221318 (C1) was taken
as the primary comparison star and all differential magnitudes reported here
are in reference to HD 220825 minus C1.


                                [FIGURE 1]
                                [FIGURE 2]


   A periodicity analysis of the differential data was undertaken utilizing
Kurtz' (1985) more efficient method of the Deeming (1975) DFT technique.
Figure 1 shows peaks in the amplitude spectrum of nearly equal height at the
frequencies .704 day^-1 and 1.708 day^-1, the amplitude of the former being
slightly greater. The second frequency corresponds to a period of 0.585 day,
essential identical with the period found by both Rakosch and van Genderen.
However, the photometric accuracy of the data obtained in this investigation is
high enough to show that in a phase plot, the frequency .704 day^-1 (period=
1.420 days) is actually the correct one and that the peak corresponding to .704
dayE-1 is actually a one-day alias in the frequency domain.
   A cosine fit to the data yield a best fit to the period 1.4200 +/- 0.0005
days; the phased data and corresponding cosine fit are shown in Fig. 2. The
rms error of this fit is just 1.8 mmag, compared with the rms error of 2.7 mmag
if one assumes a period of 0.5853 day.
   A more thorough discussion of this star, including additional photometric
data and spectroscopic results obtained by Schneider, will be presented
elsewhere.
    Acknowledgements. TJK thanks the Lowell Observatory and the National
Science Foundation (grant AST-8716971) for support of this research.


                   TOBIAS J. KREIDL

                  Lowell Observatory
                Mars Hill Rd., 1400 West
               Flagstaff, AZ 86001, U.S.A.


                   HARTMUT SCHNEIDER

             Universitatssternwarte Gottingen
                   Geismarlandstr. 11
       D-3400 Gottingen, Federal Republic of Germany



References:

Deeming, T.J., 1975. Astrophys. Space Sci., 36, 137. [BIBCODE 1975Ap&SS..36..137D ]
Kurtz, D.W., 1985. Mon. Not. R. astr. Soc., 213, 773. [BIBCODE 1985MNRAS.213..773K ]
Rakosch, K.D., 1962. Lowell Obs. Bulletin, No. 117, 227 [BIBCODE 1962LowOB...5..227R ]
van Genderen, A.M., 1971. Astron. Astrophys., 14, 48. [BIBCODE 1971A&A....14...48V ]