COMMISSION 27 OF THE I. A. U.
          INFORMATION BULLETIN ON VARIABLE STARS
                       Number 1129

                                Konkoly Observatory
                                Budapest
                                1976 May 3

               OBSERVE theta1 Ori A !*



  Photometry in an instrumental UBVR-system of another minimum
of theta1 Ori A on 1975 December 05 was made at the Bochum 61-cm-telescope 
on La Silla (Chile) using the equivalent point method described 
by Hall and Garrison (1969). The observations confirm the
preliminary period (Lohsen 1975) and, together with Strand's (1975)
two estimates lead to an improved period of 196.297 +- 0.001 days.
Since the minimum of 1974 April 25/26 depends only on one dubious
estimate, only the double value, P_0= 392.594d can be considered
as the definite fundamental period. Because of the depth of the
minima in 1975 (Walker 1976) and in 1973 and the symmetrical shape,
the minimum on 1974 November 08 was certainly not a secondary,
i.e. P =/= 2P_0. Visual estimates on 1976 March 12.75 preclude a 
period P = P_0/4 = 98.15d, too.
   Using P_0= 396.594d the observed parts of minima of three 
consecutive years can be fitted together (Fig. 1) to estimate the 
total duration D = 20 + 2 h and the constant phase d = 2.5 +- 1 h.
With the aid of the Russell-Merrill tables (1950) a ratio of the
radii k = r_s/r_g = 0.80 +- 0.05 (est.err.) and a central eclipse
(cos i < (1-k)r_g) was found. The low precision of the scanner 
observations in 1973 do not allow to decide whether primary minimum
is a transit or an occultation. In both cases it seems to be 
difficult to explain the almost insignificant color dependence of
the depth of minimum (Table 1). While the observations in 1973
might have been affected by systematic errors this explanation
seems to be improbable for the photometry in 1975. A spectral type
of B0 for the larger component is consistent with Petrie's (1965)
classification B0.5 as well as Morton and Adams' (1968) color, 
indices and an interstellar reddening of E(U-B)/E(B-V)= 0.72. If 
primary minimum is a transit, the type of the smaller component should
be later than A5, in the occultation case it should be earlier than
O5. The occultation case requires relatively small radii R_g = 3R_sun
and R_s = 2.4 R_sun in order to explain the total absolute magnitude
*Based on observations made at the European Southern Observatory.
M_V = -2.2m (m_V = 6.7m; A_V = 0.9m; DM = 8.0m). A similar small radius
of R = 2.5R_sun has been found (Hall 1971) for the brighter component
of BM Ori = theta1 Ori B, which is of spectral type B1-B3.
  Taking the radius R_g = 3.0R_sun estimated from photometry in the
occultation case, k = 0.8, and the total duration of primary minimum 
D = 20 h, we get a tangential relative velocity
        v = 2R(1+k)/D = 105 km/s = 22.15 au/y.
With a circular orbit this yields a total mass
  M_1 + M_2= r^3/P^2 = P(v/2Pi)^3= 47 M_sun (assuming P=P_0 = 1.075 y)
Considering all the uncertainties, this seems to be consistent
with the spectral types. The transit case, however, taking R_g=4R_sun
would yield a total mass of 112 M_sun, i.e. an eccentric orbit must
be assumed, which is quite common for widely separated pairs.
  Feibelman (1975a) has found two secondary minima of deltam = 0.8m
on old plates. The depth is inconsistent with the depth of primary
minimum (1.0m). Later (1975b) he corrected deltam to "considerable"
which is more in accordance with the 0.3m expected in the occultation 
case. A minimum of this depth should be detectable by visual
estimates which were made at Hamburg Observatory throughout 1974,
1975 and 1976. They failed to show a secondary on 1975 February
23 and 1976 March 21, whereas around 1975 September there seem to
have been a shallow dip. If these estimates and Feibelman's 
observations are real, the period is indeed 392.594 days and the 
orbit is considerably eccentric.
  Taken altogether, the available observations are insufficient
for a detailed planning of all further observations. As this system 
possibly allows us to measure accurately the mass and the 
radius of a young undisturbed B0 type star, every effort should be
made to observe at least the next primary minimum on 1977 January
01.1 since a similar opportunity may not occur before six or even
twelve years. Spectroscopic observations are needed especially
within some weeks before primary minimum. Apart from a desirable
continuous survey, photometry is important near the expected 
secondary minima, i.e. 1976 September 25 to October 10 and 1977 April
11 to 25.

                   Table 1
           Magnitude differences theta1 Ori A minus C

   Date             DeltaV   Delta(U-B)  Delta(B-V)  Delta(V-R)

1975 Dec.03/04     1.634+-3   0.079+-5   0.006+-4   -0.009+-3
1975 Dec.05
3.2 to 4.8 h UT    1.690      0.074  6   0.014  3   -0.014  3
4.9 to 6.8 h UT    1.865      0.092  3   0.010  3   -0.010  4
6.9 to 9.0 h UT    2.160      0.090  6   0.026  4   -0.003  5

1975 Dec.05/06     1.632+-4   0.074+-6   0.010+-4   -0.011+-3
1975 Dec.06/07     1.631  5   0.081  9   0.006  5   -0.010  4
1975 Dec.07/08     1.629  3   0.080  4   0.010  7   -0.015  5




                        ECKMAR LOHSEN
                       Hamburger Sternwarte
                       D-2050 Hamburg 80



References:

W.A. Feibelman, 1975a, AAVSO meeting 1975 October
                1975b, IAU Circ. No. 2859 [BIBCODE 1975IAUC.2859....4F ]
D.S. Hall and L.M. Garrison, 1969, PASP 81, 771 [BIBCODE 1969PASP...81..771H ]
D.S. Hall, 1971, Proc. IAU Coll. No. 15 = Kl.Veroff.Bamberg
       Vol. 9, No. 100, p. 217 [BIBCODE 1971ndnf.coll..217H ]
E. Lohsen, 1975, IBVS No. 988
J.E. Merrill, 1950, Princeton Obs. Contr. No. 23-26 [BIBCODE 1950QB4.P94n23..... ]
D.C. Morton and Th.F. Adams, 1968, Astrophys.J. 151, 613 [BIBCODE 1968ApJ...151..611M ]
R.M. Petrie, 1965, Pub. Dom. Obs. Victoria 12, 317 [BIBCODE 1964PDAO...12..317P ]
K.Aa. Strand, 1975, IBVS No. 1025
M.F. Walker, 1976, IBVS No. 1080


                        [FIGURE 1]

    Fig. 1: Magnitude differences theta1 Ori A - C