COMMISSION 27 OF THE I.A.U. INFORMATION BULLETIN ON VARIABLE STARS Number 1418 Konkoly Observatory Budapest 1978 May 2 NEW PHOTOELECTRIC MEASURES OF ETA ORIONIS The meager photometric history of the eclipsing binary Eta Ori has been described by Chambliss (1978) who also reported a few new photoelectric observations. At the Flower and Cook Observatory Eta Ori AabcB (using McAlister's (1976) notation) has been observed in yellow and blue as one unresolved source with the semi-automated, 2-channel, pulse counting Pierce-Blitzstein photometer. This multiple system is so bright that a neutral density filter, nominally diminishing the beam by 5 mag., was employed to avoid excessive statistical corrections for pulse coincidences. The simultaneously-observed comparison star was HD 35777 (B2 V); for which a nominal 2.2 mag. neutral filter was used for the same reason. The photometric scale is very closely that of the BV system but the exact neutral attenuations were not measured and the zero point of the (V-C) magnitude differences is not accurately known at this time. The counting interval was 0.0004 day and no check star was observed. The observations are listed in Table I which also enumerates the number of counts composing an observation, the internal standard deviation of that observation, and its phase calculated from the ephemeris privately provided by E. R. Zizka and W. R. Beardsley: Hel. Pr. Min. = 2415761.826 + 7.989268 E. Light time effects in the 9.174 year orbit have-been ignored for the calculations of the phases of Table I. The measures from the table are collected onto one cycle in Figure 1. Because it is known that Stromgren-y observations transform to Johnson, Morgan-V observations without great problems of photometric scale, the y-observations of Chambliss have been translated in zero point to give as small as possible a systematic difference from those of Table 1. These shifted observations also appear in the figure. The b-observations of Chambliss have not been shifted in this fashion and are not plotted. [FIGURE 1] Fig. 1. The light curve of eta Ori AabcB defined by the present yellow (large filled circles) and blue (open circles). The y observations of Chambliss are shown by the smaller filled circles. Several conclusions may be drawn at this time. At a given phase, the noise in the light curve is frequently greater than the internal precision of the observations would suggest. It is clear that this noise is not due to HD 35777. Similar noise can also be recognized in the observations of Chambliss and indicates some intrinsic variability of at least one of the stars. The light curve appears not to be flat between the eclipses, bearing out its description by Kunz and Stebbins (1916). Since, as Chambliss notes, the radii of the eclipsing members are fractionally small, this convexity of the light variation is likely to be due to a flattened distribution of circumstellar gas. Zizka and Beardsley have found ample evidence for such gas and it could be contributing to the noise in the light curve as well. The eclipses may be deeper than has been thought: 0.25 to 0.30 mag. for the primary and 0.15 to 0.25 mag. for the secondary. In view of the suspected intrinsic variability, however, this suggestion must be documented by more observations. The bandpass dependence for the eclipse depths awaits the removal of the dilution from Eta Ori AcB. This dependence is sure to be a very important correction for the present coverage of the light curve implies greater dilution for the blue than for the yellow observations, which is consistent with McAlister's interpretation of the component spectral types and absolute magnitudes. There is no doubt that the removal of the light dilution will increase the variation between the eclipses, the intrinsic variability of the system, and the eclipse ranges. As a consequence, the eclipses are fairly geometrically deep and perhaps a reasonable determinate light curve analysis will be possible. Another season of observation is planned. I am indebted to E. R. Zizka and W. R. Beardsley for sharing their results prior to publication, to W. Blitzstein for design changes and maintenance of the photometric system, and to D. E. Kjer for a night's observation. I am also grateful for the support offered by NSF grant AST 74-91656 A-01 which supported this work. ROBERT H. KOCH Department of Astronomy University of Pennsylvania Philadelphia, PA 19104, U.S.A. References: Chambliss, C. R. 1978, Comm. 27 I.A.U. I.B.V.S. No. 1398. Kunz, J. and Stebbins, J. 1916, Pub. A.A.S. 3, 272. [BIBCODE 1918PAAS....3..272K ] McAlister, H. A. 1976, Pub. A.S.P. 88, 957. [BIBCODE 1976PASP...88..957M ] Table 1. Differential Yellow and Blue Observations of Eta Ori AabcB Hel. JD - (V-C)_y n sigma Phase Hel. JD - (V-C)_b n sigma Phase 2443000. 2443000. 415.8756 -3.571 9 +0.008 0.3997 415.8759 -3.113 8 +0.004 0.3997 439.8155 3.513 14 0.012 0.3961 439.7941 3.128 12 0.019 0.3934 466.7103 3.539 15 0.004 0.7625 466.7113 3.159 16 0.003 0.7626 474.7300 3.538 20 0.005 0.7663 474.7405 3.118 21 0.003 0.7677 480.7453 3.470 22 0.004 0.5192 480.7636 3.150 22 0.006 0.5215 485.6342 3.551 16 0.002 0.1312 485.6556 3.151 10 0.006 0.1338 519.6503 3.538 10 0.011 0.3889 519.6240 3.156 19 0.005 0.3856 550.6053 3.534 15 0.003 0.2635 550.6173 3.176 16 0.019 0.2650 551.5280 3.517 18 0.003 0.3790 551.5400 3.167 14 0.002 0.3805 552.5631 3.332 12 0.005 0.5085 552.5723 3.000 12 0.005 0.5097 555.5664 3.588 20 0.009 0.8844 555.5803 3.192 20 0.005 0.8862 561.5947 3.538 5 0.011 0.6390 563.5499 3.212 11 0.005 0.8837 563.5399 3.549 11 0.002 0.8824 566.5480 3.216 12 0.002 0.2590 566.5385 3.567 13 0.003 0.2578 569.5743 3.210 13 0.002 0.6378 569.5659 3.543 12 0.003 0.6367 572.5276 -2.958 10 +0.004 0.0074 572.5188 3.302 12 0.002 0.0019 573.5096 -3.585 12 +0.002 0.1303