COMMISSION 27 OF THE I.A.U.
              INFORMATION BULLETIN ON VARIABLE STARS
                           Number 1240

                                                    Konkoly Observatory
                                                    Budapest
                                                    1977 February 15


FOUR-COLOUR PHOTOMETRY OF THE BRIGHT SOUTHERN VARIABLE HD100198

HD 100198 (HR 4438, V=6.36, A0Ia) was observed photoelectrically 
in the uvby system with the Danish National 50 cm Telescope 
and the ESO 50 cm Telescope at La Silla, Chile, on 17
nights in 1973 as comparison star for the variable O2 Cen
(HD 100262). The observations quickly revealed night-to-night
variations in all colours. Since a second comparison star was
always observed, differential measurements of HD 100198 relatively 
to HD 100380 (V=6.76, A0) are available. The observed colours 
b-y, v-b and u-v were transformed to the standard system
of Crawford and Barnes (1970). The y observations were transformed 
to the standard V magnitudes of the UBV system. In each
observing night two to ten differential measurements of each
star were made. Fig. 1 shows a plot of the nightly mean values
of y, b-y, v-b and u-v. The average nightly mean error is about
0.005 mag. in all colours.
   HD 100198 shows small irregular variations in y, b and v
with additional larger variations in u on a time scale of 
several days. The individual differential measurements obtained
during single nights were checked for short term variations,
but no indications for the presence of significant fluctuations
on a time scale of several hours was found.
   The author wishes to express his thanks to the University
of Copenhagen for the use of the 50 cm Danish Telescope and
photometric equipment.
   Part of this research was supported by the National 
Foundation of Collective Fundamental Research of Belgium (FKFO)
under contract no 2.9002.76.


                     [FIGURE 1]


                                    C. STERKEN

                              Astrophysical Institute,
                          Vrije Universiteit Brussel, Belgium
                                        and
                            European Southern Observatory,
                                  Santiago, Chile


Reference:

Crawford, D.L., and Barnes, J.V., 1970, Astron. J. 75, 978 [BIBCODE 1970AJ.....75..978C ]