COMMISSION 27 OF THE I. A. U. INFORMATION BULLETIN ON VARIABLE STARS Number 2477 Konkoly Observatory Budapest 13 February 1984 HU ISSN 0374 -0676 ON THE PERIOD OF BH CENTAURI The early type, massive contact system BH Centauri was put on a program of observations otherwise dedicated to the periods of W UMa binaries on account of recently discovered, conspicuous period variations, apparently quite different from those exhibited by other members of its group. The star has been observed on two nights at Cerro Tololo InterAmerican Observatory, in March 1981, using the No. 2 14-in. telescope and standard photoelectric equipment. 149 B and V observations define two minimum epochs: JD 2444679.663 (Min. II) 44681.6453 (Min. l) Table II gives 44 observations lying close to the minimum light. V, B are against the comparison star CPD-62d2179, the check star was CPD-62d2184 corrections for differential extinction remained under 0.001 magn. Period studies of BH Centauri are severely handicapped by a long gap of photometric observations, 1928-1967. For the history of observations and period determinations the paper Sistero, Candellero and Grieco (1982) should be consulted. In brief, a "mean value" of the period can be found, connecting, although with very large residuals, Oosterhoff's early photographic epochs with the recent photoelectric values: P_1 = 0.79158298d (Sistero, formula 3) The photoelectric observations at Cordoba Observatory indicated that the momentary period is much longer P_1 = 0.7915942d (Sistero, formula 1) Two new minima (Sistero et al.) lead to a somewhat shorter period while the Cerro Tololo observations, combined with the two earlier series of photoelectric minima, suggest the best fitting period value: P_3 = 0.79159441d virtually identical with the period in Sistero's formula 1. Table I BH Centauri, photoelectric min. epochs Min I=JD2444028.5796+0^d.79158298E JD2439621.7975 I O-C=-0.0397d Leung, Schneider 1977 43987.8119 II -0.0012 Sistero et al.1979 43989.7917 I -0.0003 " 43990.5835 I -0.0001 " 44028.5796 I 0.0 " 44095.4693 II +0.0009 " 44280.7071 II +0.0023 Sistero et al. 1982 44429.5212 II +0.0043 " 44679.6663 II +0.0079 This paper 44681.6453 I +0.0079 " All available photoelectric O-C determinations are listed in Table I. The minimum epoch obtained by Leung and Schneider yields a very large negative residual of nearly 1 hour, the epoch itself seems, however, beyond doubt correct. Based on the relatively short time span of these observations, a surprisingly large value was derived for the period (0.791616d) which is in conflict with all later data. Recognition that the period has changed considerably follows from a comparison of P_1 and P_2 or P_3. [FIGURE 1] Figure 1 Timing residuals for BH Cen. Photographic data are (in part) normal points. "L-Sch" means the minimum epoch by Leung and Schneider at O-C = -0.040d. The broken circle refers to an apparent shift of the minima found by Eggen, assuming that these observations were done in Spring 1976 (cf. Sistero' et al. 1982). The O-C diagram of the minimum epochs (Figure 1) illustrates the background of Sistero's interesting proposal of a possible light-time effect. There is, indeed, a similarity in the system's behaviour, shown in the 1920s and 1970s. Clearly, further and continued observations are needed but the hypothesis of a third body in the system can be subjected to an order of magnitude test of reliability. (See, for example, Irwin's and Landolt's discussion of SV Centauri 1972, or the introduction to the paper of Frieboes-Conde and Herczeg, 1973.) Table II Observations JD2444679+ delta_V JD2444679+ delta_B .6496 -0.195 .6502 -0.195 .6532 -0.261 .6542 -0.297 .6548 -0.272 .6562 -0.305 .6571 -0.298 .6601 -0.371 .6608 -0.352 .6624 -0.375 .6630 -0.352 .6660 -0.397 .6667 -0.352 .6679 -0.373 .6692 -0.343 .6726 -0.314 .6732 -0.297 .6777 -0.284 .6788 -0.237 .6813 -0.228 .6819 -0.195 .6834 -0.194 .6840 -0.148 JD2444681+ delta_V JD2444681+ delta_B .6288 -0.245 .6293 -0.218 .6326 -0.306 .6336 -0.287 .6351 -0.337 .6358 -0.315 .6396 -0.394 .6400 -0.361 .6414 -0.407 .6420 -0.410 .6457 -0.409 .6463 -0.404 .6480 -0.389 .6486 -0.383 .6520 -0.374 .6525 -0.333 .6546 -0.349 .6551 -0.307 .6605 -0.255 .6610 -0.212 The amplitude of the light time variation can only be most crudely estimated , it may be perhaps 1 2/3 hours, that is, for small eccentricity a_1 sini. = 6 AU. Assuming the period P = 50 years and M_1 + M_2 = 20M_sun for the eclipsing pair, we obtain the `following masses for the hypothetical third component in the system: if i = 90d, M_3 = 3.6M_sun i = 60d, M_3 = 4.3M_sun i = 30d, M_3 = 8.2M_sun However preliminary and uncertain these estimates might be, the resulting masses are acceptable values and they, at least, do not contradict the light time hypothesis. A distance to the system of 2 kpc would Lead to a possible angular separation of the order of 0.01" to 0.02", Partial support of this research program by the National Science Foundation (Washington, D.C.) is gratefully acknowledged as well as Ms. Linda Barker's cooperation in the reduction of observations and R.P. Olowin's assistance in editing the manuscript. TIBOR J. HERCZEG Department of Physics and Astronomy University of Oklahoma, Norman, Oklahoma References: Frieboes - Conde, H. and Herczeg, T. 1973, AAp. Suppl. 12, 1 [BIBCODE 1973A&AS...12....1F ] Irwin, J.B. and Landolt, A.D. 1972, PASP 84, 686 [BIBCODE 1972PASP...84..686I ] Leung, K.C. and Schneider, D.P. 1977, Ap J 211, 844 [BIBCODE 1977ApJ...211..844L ] Sistero', R.F., Candellero, S.A., Grieco, A., 1982 Bol.Asoc.Arg.Astron. 27, 68 [BIBCODE 1982BAAA...27...67S ] [From IBVS 2526] CORRIGENDA (to I.B.V.S. No. 2477) On p.1. The observed time of Min. II is J.D. 2444679.6663. (The J.D. of this minimum is correctly stated in Table I.) On p.2. Table I. The O-C value given for the last two minima is +0.0097 and not +0.0079. (These residuals are correctly plotted in Fig.1.)