COMMISSION 27 OF THE I. A. U.
      INFORMATION BULLETIN ON VARIABLE STARS
                   Number 1070
          

                             Konkoly Observatory
                             Budapest
                             1975 November 29
         


           ON THE PERIOD AND ORBIT OF THE
           ECLIPSING BINARY Theta^1 Ori A

Information Bulletin No. 988 by E. Lohsen announced
the variability of Theta^1 Ori A, with a period of 196.25 days
and a one magnitude drop during eclipse which lasted not
more than 24 hours. Lohsen mentioned the possibility of
a period 1/3 or 1/2 of the above value.  Subsequently,
Strand confirmed the period to be 196.298 days (I.A.U.)
Information Bulletin No. 1025), based on Dearborn, Lowell
and U. S. Naval Observatories data, but was unable to
find a shorter period or secondary minimum at +- 1 day
from the mid-point of the 196.298 day period.
Following these announcements, some 60 Allegheny
plates taken with the 76cm refractor between 1963 and 1969
were searched for variability.  Visual inspection showed
that on 3 nights Theta^1 Ori A was fainter than Theta^1 Ori D.
Variability of Theta^1 Ori A was then confirmed by means of an
ANTECH TV-Densitometer and by measuring image diameters
with a Gaertner machine. The relevant data for Theta^1 Ori A
and Theta^1 Ori D are:

Jan 3-4, 1966                Delta m = 0.8m +- 0.1
Jan 4-5, 1966                Delta m = 0.4m +- 0.1
Jan 30-31, 1967              Delta m = 0.8m +- 0.1

These data are interpreted as secondary minima that are
wider and probably shallower than the primary minima,
occurring about 9 days after the mid-point of the 196.298
day period and explain why they were not found by Strand.
The displaced, wider secondary minimum strongly suggest
an eccentric orbit for Theta^1 Ori A.
Lohsen and Strand predict the next (primary) minimum
to occur on Dec. 5, 1975 and to last for ~24 hours. The
next secondary minimum should occur near March 20, 1976
and should be observable for at least 2 nights.
Observations of Theta^1 Ori A, including radial velocities,
for a duration of at least a week centered on these dates
appear desirable.



                     WALTER A. FEIBELMAN
                NASA, Goddard Space Flight Center