COMMISSIONS 27 AND 42 OF THE IAU
              INFORMATION BULLETIN ON VARIABLE STARS
                          Number 3945

                                               Konkoly Observatory
                                               Budapest
                                               19 October 1993

                                               HU ISSN 0374 - 0676


       A NULL DETECTION OF RAPID OSCILLATIONS IN THE Ap STAR ET And


  Panov (1985) reported the possible detection of rapid oscillations in the CP2 (Ap) star ET
And (HD 219749, HR 8861), a B9p variable with a rotation period of approximately 1.62 days
and also a short-period pulsational variable with periods most recently determined to be 0.09919
and 0.14795 day (Weiss et al. 1994). Panov's claimed detection of periods between 7 and 16 min
prompted the need for additional observations to look for rapid oscillations.

  Photoelectric photometry was obtained on three consecutive nights with the Lowell Observatory 
0.8-m reflector. The journal of observations is given in the table below. Each integration was
20 sec through a narrow-band (4060A, FWHM= 70A) filter with a central wavelength similar to
Stromgren v. Breaks were taken only for sky measurements and recentering of the 20" aperture.


  Table 1. Journal of observations of ET And.
  ---------------------------------------------
  UT date  Julian day   Npts sigma  Delta t(hr)
  ---------------------------------------------
  93-Sep-21 2449251     503  .0023       2.89
  93-Sep-22 2449252     532  .0022       3.09
  93-Sep-23 2449253     539  .0024       3.11
  ---------------------------------------------
                Summa 1574 <.0028> Summa 9.09
-----------------------------------------------

  The times of observation were transformed to heliocentric Julian Day to 1-sec accuracy; and
mean magnitudes were subtracted from each night's data after counts were corrected for dead time
and sky-subtracted.

  Analysis was performed via discrete Fourier transform (DFT) using Kurtz' (1985) modified
version of Deeming's (1975) algorithm. Fig. 1 shows the amplitude spectrum of each night 
separately, and in the bottom panel, the combined data for all three nights. It should be emphasized
that here, the y-scale is in milli-magnitudes, so the height of each separate panel is approximately
1.0 mmag. In particular, in the frequency range 90-205 c/d, corresponding to periods of 7-16 min.
note the absence of any significant power. The upper limit is about 0.6 mmag for any individual
night. It was about 0.45 mmag for all three nights combined. This is also the 5% false alarm
probability level (Scargle 1982; Horne and Baliunas 1986).

   Given that rapid oscillations in Ap stars are proportional to the magnetic field strength, the
phase of the magnetic field and hence the star's rotation phase can play a significant role. With a
rotation period of 1.61887 days (Scholz et al. 1985), it can readily be 
calculated that these data
were obtained at relative phases of 0.0; 0.10, and 0.71 and so the maximum gap in phase is about
0.6. This precludes completely missing any more than about half the portion of the the phase
where the magnetic field strength is at a maximum, and even less if there is magnetic polarity
reversal. From only two magnetic field measurements published by Bohlender et al.(1993), it is
unclear whether polarity reversal takes place or not.

                               [FIGURE 1]

 Fig 1.  Amplitude spectra of the data from Table 1 for each separate night, and at the bottom,
for all three nights combined.

   We conclude that there is no evidence of rapid variability in ET And, and that if data are not
obtained under excellent photometric conditions, spurious peaks in the period range reported by
Panov (1985) could easily show up.


                                TOBIAS J. KREIDL

                                Lowell Observatory
                                Mars Hill Rd., 1400 West
                                Flagstaff, AZ 86001, U.S.A.
                                Internet: tjk@lowell.edu


References

Bohlender, D.A., Landstreet, J.D. and Thomson, I.B., 1993, Astron. Astrophys., 269, 355. [BIBCODE 1993A&A...269..355B ]
Deeming, T.J., 1975, Astrophys. Space Sci., 36, 137. [BIBCODE 1975Ap&SS..36..137D ]
Horne, J.H., and Baliunas, S.L., 1986, Astrophys. J., 302, 757. [BIBCODE 1986ApJ...302..757H ]
Kurtz, D.W., 1985, Mon. Not. R. astr. Soc., 213, 773. [BIBCODE 1985MNRAS.213..773K ]
Panov, K.P., 1985, IBVS, No. 2825.
Scargle, J., 1982, Astrophys. J., 263, 835. [BIBCODE 1982ApJ...263..835S ]
Scholz, G., Gerth, E., and Panov, K.P., 1985, Astron. Nachrichten, 306, 329. [BIBCODE 1985AN....306..329S ]
Weiss, W.W., Kuschnig, R., Kreidl, T.J., Bus, S.J., Osip, D.J, Guo, Z., Hao, J., Huang, L.,
  Sareyan, J.-P., Alvarez, M., Bedolla, S.G., Zverko, J., Ziznovsky, J., Panov, K.,
  and Polosukhina, N., 1994, Astron. Astrophys., submitted.