COMMISSION 27 OF THE I.A.U.
              INFORMATION BULLETIN ON VARIABLE STARS
                           Number 1211

                                             Konkoly Observatory
                                             Budapest
                                             1976 December 1


     A TENTATIVE SPECTROSCOPIC ORBIT OF Theta1 Ori A *

Three out of four spectrograms of Theta1 Ori A = HD 37020 taken
by M. de Groot (ESO) at the 1.5 m telescope at 12.3 A/mm dispersion 
show the Balmer absorption lines clearly blue-shifted against 
the nebular emission lines. Two of the spectra, 524 d
apart, suggest a new period of 130.864 d, i.e. one third of the
"fundamental" period of 392.594 d (Lohsen 1976 IBVS 1129). The
former period has to be halved according to observations by
M. Baldwin (1976 IAU Circ. 3004). Fig. 1 shows our observed radial
velocities together with older ones, using the period 65.43233 d
and the primary minimum of 1977 JAN 01, 02.2h = JD 2443 144.600
as phase zero.
    The curve represents an estimated orbit based on
e = 0.60, omega = 180deg, K1 = 33.3 km/s, and gamma = 13.3 km/s + V_neb.
Together with a photometrically estimated relative tangential
velocity of 140 km/s (=K1+K2 if omega=180 deg) at primary minimum in
the transit case this implies the masses to be

       M1 = 1.036.10^-7(K1+K2)^2 K2P(1-e^2)^3/2= 7.3 M_sun
   and M2 = M1K1/K2 = M1/3.2 = 2.3 M_sun.

The low mass ratio explains why the colours of the system do
not change significantly during primary minimum and makes it
improbable to detect the companion spectroscopically. Better
light and RV curves, however, would supply us with a rigid
mass-radius relation for both components. The radius may be
obtained by photometric comparison with BM Ori, where it can
be measured directly (Popper and Plavec 1976 ApJ 205, 462).
   An interesting implication of this orbit of Theta1 Ori A is
that it reduces significantly the kinetic energy of the Trapezium
system in Parenago's calculation (1954 Trudy Sternberg Inst.
Vol. 25) and together with the absence of measurable relative
tangential motions (Allen and Poveda 1974 Rev. Mexicana Astr.
Ap. 1, 101) makes the system stable within the observational
errors.
            
                      [FIGURE 1]


* Based on observations made at the European Southern 
  Observatory.


                                ECKMAR LOHSEN

                             Hamburger Sternwarte
                              D-2050 Hamburg 80