COMMISSIONS 27 AND 42 OF THE IAU
               INFORMATION BULLETIN ON VARIABLE STARS
                            Number 4069

                                                 Konkoly Observatory
                                                 Budapest
                                                 16 August 1994
 
                                                 HU ISSN 0374 - 0676


         DISCOVERY OF PULSATION IN THE LAMBDA Boo STAR HD 111786



 The group of lambda Bootis stars consists of early A type population I objects which are metal
poor and their v * sin i values exceed on average 100 km/s. HD 111786 (m_V=6.15, HR 4881,
BD-26d9369) was classified by Gray and Corbally (1993) as A1.5 Va lambda Boo.
  Our photometry was obtained using the `modular photometer' attached to the 0.5m telescope
of the South African Astronomical Observatory (SAAO) in Sutherland and was scheduled from
April 20, to May 10, 1994. The measurements are 10 second integrations with a Stromgren v
filter. A 30 arcsec diaphragm was used throughout. As primary comparison star we used
   HD 111295 (C1, m_V=5.7, HR 4860, BD-26d9340, G5III-IV), and during the last two
nights as an additional comparison star
   HD 111226 (C2, m_V=6.4, HR 4857, BD-26d10540, B8III).

  HD 111786 was found to be variable already in the first night (May 3/4, 1994) and monitored
during the following seven nights. A full observing log is given in Table 1. The observations
from the night of May 8/9 were excluded from our present analysis, because of strong sky
transparency variations and hence poor quality of the data.


           Table 1: Observing log.

night     hours               data points
May '94             HD111786    HD111295   HD111226
3/4        3.62         1118          69          -
4/5        7.80         2591          57          -
5/6        7.43         2362          72          -
6/7        6.51         1916          57          -
7/8        7.28         2377          72          -
8/9        5.15          734         201        202
9/10       7.38         1550         216        211
total     45.17        12648         744        413


  Figure 1 shows the light curves of the program and both comparison stars for the last
night. The instrumental magnitudes are plotted relative to the night mean. The variability of
HD 111786 is clearly visible and superimposed on low frequency transparency changes, which
are evident also in the light curves of both comparison stars.
  In a next step, long term trends were removed from the data sets of each night and a Fourier
analysis was computed. The amplitude spectra and the spectral window from the merged data
are shown in Fig. 2. The maximum peak in the frequency spectrum appears at f=31.02 d^-1
(46.42 min) with a semi-amplitude of 6.2 mmag. This amplitude spectrum gives evidence for
the presence of more than one pulsation frequency, a presumption which is supported by the
amplitude modulation of the entire light curve (Figure 3).


                             [FIGURE 1]

  Figure 1: Instrumental Stromgren v data for HD 111786 and both comparison stars

                             [FIGURE 2]

  Figure 2: Amplitude spectra (v) for HD 111786 and the comparison star C1

                             [FIGURE 3]

               Figure 3: Light curve of HD 111786 


  This object is the third one found to be variable as a result of our survey for pulsation among
lambda Boo stars (Paunzen & Weiss 1994, Weiss et al. 1994). The driving argument for this survey
was the possibility to apply the technique of asteroseismology for determining the structure and
evolutionary status of lambda Boo stars. Finding multi-periodic pulsation increases significantly the
potential of this method, because each new observed frequency allows a better determination of
stellar model parameters.


  Acknowledgement: This research was supported partly by the Osterreichische Akademie der
Wissenschaften (project Asteroseismologie), and by the Hochschuljubilaumsstiftung der Stadt
Wien.



                         Rainer KUSCHNIG
                         Ernst PAUNZEN
                         Werner W. WEISS
                         Institut fur Astronomie
                         Turkenschanzstr. 17
                         1180 Wien
                         Austria
                         e-mail: familyname@astro.ast.univie.ac.at


References:

Gray R.O., Corbally C.J., 1993, AJ, 106, 632 [BIBCODE 1993AJ....106..632G ]
Paunzen E., Weiss W.W., 1994, IBVS, No. 3986
Weiss W.W., Paunzen E., Kuschnig R., Schneider H., 1994, A&A, 281, 797 [BIBCODE 1994A&A...281..797W ]