COMMISSION 27 OF THE I. A. U.
         INFORMATION BULLETIN ON VARIABLE STARS
                      Number 1025

                                Konkoly Observatory
                                Budapest
                                1975 July 31
    

              ON THE VARIABILITY OF Theta^1 Ori A


    The announcement of E. Lohse in the Information Bulletin No.
998 of the variability of Theta^1 Ori A resulted in a search for 
observed minima among the material of multiple exposure plates taken
of Theta^1 Orionis between 1959 and 1965 with the visual refractors of
the Dearborn, Lowell and U.S. Naval Observatories. The plates were
taken for the purpose of determining the relative positions of the
four components of the Trapezium.
    Plates taken within 2 days of the minima as determined from
the Lohse period of 196.25 +-0.1 days were examined for possible 
observations of such events. Among these plates 2 Flagstaff plates
taken by O. Franz on 21 October 1959 showed the star in question
diminished in brightness by 0.7 magnitude at 10h 58m UT, The first
plate was taken with an objective grating providing a magnitude
difference of two between central image and first order spectra,
allowing fairly close estimate of a DeltaV = 2.2 between components
A and C. The second plate, taken without a grating at 11h 18m UT.,
shows component A even fainter, making DeltaV = 2.5.
    The MJD for this observation is 36862.470d while the MJD for
Lohse's observation on 11 October 1973 is 41966.229, assuming a
UT of 5h 30m for DeltaV = 2.5 on his light curve. On the basis of these
data, we obtain a period of 196.298+-0.002 days, based upon an 
estimated uncertainty in the Flagstaff DeltaV value.
   A plate taken at the Dearborn Observatory on 2 March 1954
might have covered the secondary minimum based upon the Lohse 
period. No decrease in magnitude was observed and the revised period
makes that event occur 1.8 days earlier.
   On the basis of the period derived here the next eclipse
minimum will take place on 1975 Dec. 5.484 (11h 37m UT), 
beginning approximately 12 hours earlier if the duration of the
minimum is 24 hours.




                   K.Aa. STRAND
                   U.S. Naval Observatory
                   Washington, D.C. 20390. U.S.A.




References:

K. Aa. Strand 1959, Dearborn Observatory Annuals, 7, Part 2.
K. Aa. Strand 1969, Publ. U.S. Naval Observatory, 18, Part 5. [BIBCODE 1969PUSNO..18.....S ]