COMMISSIONS 27 AND 42 OF THE IAU
             INFORMATION BULLETIN ON VARIABLE STARS
                         Number 3969

                                                Konkoly Observatory
                                                Budapest
                                                4 January 1994

                                                HU ISSN 0374 - 0676


              OPTICAL LIGHT VARIABILITY OF LS 3074


  We report detection of optical light variations of LS 3074, a luminous object in the
southern coal-sack zone (Marraco and Orsatti, 1982).
  Morrell and Niemela (1990) discovered its binary nature and derived preliminary orbital
elements based on the velocity curves of both components which were classified as O4f+
and O6-7(:). The very low values of M*sin^3i for both components (8 and 9 M_Sun, later
on corrected to 9.5 and 10 M_Sun by Niemela et al. (1992) would imply a low inclination
of the system. But trying to determine the inclination by means of linear polarization
measurements Niemela et al. (1992) derived a value of 75d. This is by far too high to put
the masses at the expected level about 20-30 M_Sun (Herrero et al., 1992). That procedure
of fixing the inclination was questioned by the authors themselves because it could be
affected by additional polarization in the system.
  Since empirical masses of early O type stars are very poorly known it seemed 
worthwhile to look for photometric variations of LS 3074 to possibly get a better estimate of
the inclination. During an observing stay at the ESO Observatory La Silla/Chile at the
beginning of April 1991 some UBVRI measurements could be obtained using the standard
single-channel photometer at the ESO 50 cm telescope. Folding the data with the 
spectroscopic period (P=2.185 d) given by Niemela et al. (1992) clearly revealed ellipsoidal
variations. Figure 1 shows the resulting V light curve with a full amplitude of about
0.15 mag. Open squares represent measurements obtained under less favourable weather
conditions. Their estimated error amounts to about 0.04 mag whereas the error of the
more reliable measurements (filled squares) is around 0.02 mag. For comparison with
UBV values compiled by Marraco and Orsatti (1982) we give here colours measured at
V=11.74 (11.73): B-V=1.13 (1.17), U-B=-0.10 (-0.10), V-R=0.81, V-I=1.59 (data
by Marraco and Orsatti in brackets). The values are affected by the strong reddening
towards the coal-sack region.
  Preliminary photometric elements were determined using the GRADUS-code (Simon
et al., 1994), a light curve synthesis program which employs Roche geometry and allows
a careful treatment of heating and scattering effects. Instead of a limb darkening law the
angular distribution of radiation generated by non-LTE model atmospheres is applied.
With the fixed temperature of 46 000K for the O4f+ star and 40 000K for the O6-7 star
(Kudritzki and Hummer, 1990), and with the fixed spectroscopic parameters (radial 
velocity amplitudes (222 and 218 km/s) and M x sin^3i) we found reliable light curve fits
for inclinations around 50d. This yielded (nearly identical) masses for both components
around 20-21 M_Sun and polar radii around 8.7-8.8 R_Sun. With these parameters LS 3074 
constitutes a contact system. Increasing the inclination results in a synthetic light curve
showing first evidence for a grazing eclipse which cannot be recognized in the present
photometric data. Nevertheless, this might unveil itself in high quality measurements.
The fit for e.g. an inclination of 55d is still acceptable (but worse than that for 50d) so

                                [FIGURE 1]

 Figure 1. V light curve of LS 3074. Phases were calculated according to the
ephemeris HJD 244 8348.619+2.185d X E. For further explanation see text.

that at present no clear decision can be made. The corresponding parameters are: 17-
18 M_Sun, 7.6-8.3 R_Sun, semidetached system. For inclinations below 50d no acceptable fit
to the light curve could be found. We conclude that inclinations between 50d and 55d
represent best the spectroscopic and photometric measurements available at the moment
thus restricting the masses of LS 3074 to about 17-21 M_Sun. This result is in line with the
masses for galactic O stars derived from spectroscopic analysis by Herrero et al. (1992)
which are systematically lower than those obtained from evolutionary calculations.
 Better spectroscopic (spectral types, radial velocity amplitudes) as well as photometric
(grazing eclipse) data is needed to analyse this system with the high accuracy appropriate
to the importance of the problem in question.


                          R. HAEFNER
                          K. P. SIMON
                          A. FIEDLER
                          Universitats-Sternwarte
                          Scheinerstrasse 1
                          81679 Munchen
                          Germany



References:

Herrero, A., Kudritzki, R. P., Vilches, J. M., Kunze, D., Butler, K., Haser, S., 1992,
   Astron. Astrophys., 261, 209 [BIBCODE 1992A&A...261..209H ]
Kudritzki, R. P., Hummer, D. G., 1990, Ann. Rev. Astron. Astrophys., 28, 303 [BIBCODE 1990ARA&A..28..303K ]
Marraco, H. G., Orsatti, A. M., 1982, Rev. Mezicana Astron. Astrof., 5, 183 [BIBCODE 1982RMxAA...5..183M ]
Morrell, N. I., Niemela, V. S., 1990, ASP Conf. Series, Vol. 7, 57 [BIBCODE 1990ASPC....7...57M ]
Niemela, V. S. Cerruti, M. A., Morrell, N. I., Luna, H. G., 1992, IAU Symp., No. 151, 505 [BIBCODE 1992IAUS..151..505N ]
Simon, K. P., Pfeiffer, M., Fiedler, A., 1994, Astron. Astrophys., in prep.