COMMISSION 27 OF THE I. A. U. INFORMATION BULLETIN ON VARIABLE STARS Number 1048 Konkoly Observatory Budapest 1975 October 3 A PRELIMINARY REPORT ON THE LIGHT VARIATION OF THE EXTREME A-SUPERGIANT CD-33d12119 (=3U1727-33?) 1. Introduction The extreme A2 supergiant CD-33d12119 (m_v=10.2 mag, A2Ia) has been suggested to be the optical counterpart of the compact X-ray source 3U1727-33 (Krzeminski and Garrison, 1973; Hensberge and Zuiderwijk, 1975). In this note we present a preliminary reduction of photoelectric observations of this star, which have been made with the Walraven five-colour simultaneous photometer, attached to the 90-cm Light-Collector of the Leiden Southern Station (at the SAAO-Annexe, formerly the Republic Observatory Annexe) Hartebeestpoortdam, South Africa. A description of the photometer and the photometric system has been given by Walraven and Walraven (1960) and by Rijf, Tinbergen and Walraven (1969). We only give here a light-curve in the V-passband. 2. The Observations The observations were made in the period May 10 - August 13, 1975. The comparison star used was a 9.2 mag. B-star HD 208883. The integration time for one observation was 0.5 or 1 min. Fig.1 shows the brightness of CD-33d12119 minus comparison star in log intensity for the V-passband. The observational error is about 0.005 in log intensity. The observations reveal that there is a variation on a long time-scale. This variation appears like a double wave on a time-scale of about 75 days and might indicate that we are possibly dealing here with a binary system. We will continue our observations of the star to study this variation in more detail. Unfortunately, the present observations are contaminated with a few percent extra light due to straylight of a 13 mag. companion star (at a distance of about 15 seconds of arc). This was due to a sudden shift of the diaphragm with respect to the grid in the oculair, over a distance of a few seconds of arc into the direction of this faint companion. Dr. P.R. Warren who could not confirm the supposed flare activity of this star (van Genderen, 1975), suggested to us the possibility of a faint companion star in or near the diaphragm (Warren, 1975). 3. Discussion of the Results We derive for CD-33d12119 the following mean brightness and colour index in the Johnson UBV-system: V_J =~ 10.25m and(B-V)_J=~1.9m, in good agreement with the results of Krzeminski and Garrison (1973). Assuming the star to be a normal A2 supergiant, we calculate a blue (interstellar) absorption of A_BJ =~ 8.5 mag. from the Walraven V-B/B-U diagram of standard stars. For the comparison star HD 208883 we derive A_BJ = 1.3 mag. The reddening of CD-33d12119 may be partly of interstellar and partly of circumstellar origin (e.g. due to circumstellar dust). Taking this reddening into account, the star is much more luminous in the Walraven L-passband than normal A-supergiants. This might be due to the fact that the hydrogen absorption lines and the Ca II (K and H) lines are less deep than in normal A-stars (Hammerschlag-Hensberge et al., 1975), due to partial filling in by emission at the red side of these lines. Assuming that the A-supergiant has a mass of 25 M_Sun and a radius of 1 AU (cf. Wolf, 1972, 1973; Lamers, 1974), we derive a lower limit for the orbital period of a possible companion star of P >= 73 days. This might support our suggestion that the long term variation (over 75 - 80 days) observed in the light-curve, is due to gravitational deformation of the A-supergiant by a compact companion star. A.M. VAN GENDEREN Leiden Southern Station P.O. Box 13, Broederstroom 0240 Transvaal, South-Africa, and G. HAMMERSCHLAG-HENSBERGE Astronomical Institute University of Amsterdam Roetersstraat 15, Amsterdam, The Netherlands. [FIGURE 1] Fig. 1. The brightness of CD-33d12119 (=3U1727-33?) minus comparison star (in log intensity) plotted against the South African calendar dates for V-passband. References: van Genderen, A.M. 1975, IAU Circ. No. 2814 [BIBCODE 1975IAUC.2814....3V ] Hammerschlag-Hensberge, G., Zuiderwijk, E.J., van der Heuvel, E.P.J. 1975, in preparation Hensberge, G., Zuiderwijk, E.J. 1975, IAU Circ. No. 2791 [BIBCODE 1975IAUC.2791....3H ] Krzeminski, W., Garrison, R.F. 1973, IAU Circ. No. 2569 [BIBCODE 1973IAUC.2569....1K ] Lamers, H.J. 1974, paper presented at the Discussion Meeting on Astronomy in the UV, Organised by the Royal Society, London. Rijf, R., Tinbergen, J., Walraven, Th. 1969, Bull.Astr.Inst.Netherl. 20, 279 [BIBCODE 1969BAN....20..279R ] Walraven, Th., Walraven, J.H. 1960, Bull.Astr.Inst.Netherl. 15, 67 [BIBCODE 1960BAN....15...67W ] Warren, P.R. 1975, private communication Wolf, B. 1972, Astron.Astrophys, 20, 275 [BIBCODE 1972A&A....20..275W ] Wolf, B. 1973, Astron.Astrophys, 28, 335 [BIBCODE 1973A&A....28..335W ]