COMMISSION 27 OF THE I.A.U.
              INFORMATION BULLETIN ON VARIABLE STARS
                           Number 1274

                                                    Konkoly Observatory
                                                    Budapest
                                                    1977 May 9


    UBV OBSERVATIONS OF A PRIMARY ECLIPSE OF Theta1 Ori A

Photoelectric UBV photometry of of Theta1 Ori A = ADS 4186A relative
to component C was carried out on the night of 1977, March 6/7 with
an area-scanning photometer (Franz 1970) on the 72-inch (1.8-m)
Perkins telescope at the Lowell Observatory. A refrigerated EMI 9526
photomultiplier was used in conjunction with standard UBV filters.
The scanning aperture was a slit of 5-mm (32-arcsec) length and
100-micron (0.64-arcsec) width. Proper choice of the effective scan
length (25 arcsec), scan direction (approximately N to S), and placement 
of the slit permitted simultaneous, separate measurement of
components A, B, C, and E, while excluding stars D and F from the
record.
   The observations, consisting of 10-scan integrations in each
color made at a sweep-speed of one scan per second, yielded one set
of UBV data every three minutes. Three-point averages of the resulting 
UBV magnitude differences for Theta1 Ori (A-C) and the corresponding
times of observation (U.T.) are listed in Table 1 and plotted in
Figure 1. Since Theta1 Ori A was still at full light when observation
began, these measures cover a portion of the light curve, including
the point of first contact which appears to have occured at
3h25m +- 3m (s.e.) U.T., that had not been previously defined by
(published) photoelectric data. By folding these new observations
onto the mean light curve published by Lohsen (1976), one finds a
period P = 65.4325 +- 0.0002 days, in excellent agreement with the
65.43-day period recently determined by Baldwin (1976).
   Interestingly enough, while Theta1 Ori A was entering eclipse,
Theta1 Ori B = BM Ori was exiting from eclipse, undergoing a brightness
increase of 0.6 and 0.7 mag in V and B, respectively, during the
3.8-hour interval of observation. The photometric data for BM Ori
will be published separately in the near future.


   This work was carried out with the support of grants from the
National Science Foundation.

                                      OTTO G. FRANZ

                                      Lowell Observatory
                                      Flagstaff, Arizona 86002
                                      U.S.A.



References:

Baldwin, M. 1976, IAU Circ. No 3004 [BIBCODE 1976IAUC.3004....1M ]
Franz, O. G. 1970, Lowell Obs. Bull. No 154 [BIBCODE 1970LowOB...7..191F ]
Lohsen, E. 1976, IBVS No 1129




                  Table 1.
      UBV Magnitude Differences Theta1 Ori (A-C)
                 1977, March 13

U.T.     DeltaV  DeltaB  DeltaU  U.T.         DeltaV     DeltaB DeltaU
02h31m   1.637   1.626   1.725   05h05m       1.704      1.725  1.799
   39    1.637   1.631   1.701      13        1.731      1.723  1.822
   47    1.628   1.637   1.710      21        1.738      1.758  1.827
   55    1.646   1.636   1.712      29        1.746      1.772  1.852
03 04    1.629   1.632   1.722      39        1.774      1.772  1.855
   11    1.640   1.630   1.735      48        1.766      1.770  1.898
   18    1.641   1.639   1.708      56        1.804      1.801  1.862
   26    1.636   1.647   1.708   
   36    1.652   1.638   1.721   06 05        1.810      1.814  1.921
   43    1.646   1.636   1.727      13        1.836      1.823  1.989
   49    1.661   1.650   1.725
   57    1.646   1.645   1.719

04 06    1.656   1.644   1.739
   14    1.658   1.657   1.739
   22    1.684   1.666   1.750
   30    1.682   1.678   1.760
   40    1.697   1.686   1.788
   49    1.702   1.701   1.784
   57    1.718   1.708   1.785


                             [FIGURE 1]

    Figure 1. UBV magnitude differences Theta1 Ori (A-C).