COMMISSION 27 OF THE I. A. U. INFORMATION BULLETIN ON VARIABLE STARS Number 3139 Konkoly Observatory Budapest 28 January 1988 HU ISSN 0374-0676 UBVRI OBSERVATIONS OF THE CHROMOSPHERICALLY ACTIVE STAR HD 155555 (V824 Ara) IN 1986 AND 1987 The RS CVn-type binary with the memorable name HD 155555 (now called V824 Ara) has a relatively short orbital period P = 1.6817 days and consists of components of similar mass (Bennett et al., 1962, Stacy et al., 1980, Fernandez- Figueroa et al., 1986). Its optical variability was described by Udalski and Geyer (1984) who found a typical spotted-star migration-wave with the amplitude of about 0.08 in V. The rotation period implied by the spot re-appearance was found to be 1.66 days. Udalski and Geyer gave references to the literature before 1984. The subsequent photometric publications were by Bopp et at., (1986) (UBV data obtained in 1985), Lloyd Evans and Koen (1987) (UBVRcIc data from 1979 and 1981), and Collier Cameron (1987) (UBVRcIc data from 1979 and 1980). The present paper contributes to the monitoring of the optical variability of HD 155555. The observations were obtained in June 1986 and in March 1987 at the Las Campanas observatory using the 61 cm telescope of the University of Toronto and the single-channel photometer. The E regions were observed to place the observations in the Cousins UBVRcIc system. Continuously poor photometric conditions prevailed during both runs and it is rewarding that the data for comparison stars do not drastically differ from those obtained by previous observers. There may exist some systematic differences for the B - V and U - B colours of the comparison stars, most probably due to the redness of both stars because mean colours for the program star do not show any systematic deviations. The VRcIc data seem to be in good accord with the previous observations. The mean data for the comparison stars are given in Table I. Each observation of HD 155555 was referenced to both comparison stars and a mean value was taken. These mean values are listed in Table II. The phases in that table have been computed using the ephemeris for the light minimum given by Udalski and Geyer (1984): Min. (JD hel.) = 2445803.07 + 1.66 E The observations for both years are shown in the graphical form in the accompanying figure. As we can see there, the time of minimum light was observed to agree with the ephemeris in 1986 but was shifted by about a half of the period in 1987. This shift can be due either to a spot migration or to an accumulated uncertainty in the rotational period. The approximate moments of the light minima in 1986 and 1987, which - in future - may help to define better the variability characteristics of HD 155555 are: June 1986: Min. (JD hel.) = 2446583.30 March 1987: Min. (JD hel.) = 2446867.95 Table I THE COMPARISON STARS FOR HD 155555 U - B B - V V V - Rc V - Ic Reference HD 156427 1.656 1.494 7.395 0.805 1.543 Udalski and Geyer (1984) 1.660 1.492 7.410 Bopp et al.,(1986) 1.50 7.42 0.80 1.53 Collier Cameron (1987) 1.695 1.453 7.417 0.797 1.555 Las Campanas 1986 1.712 1.469 7.419 0.790 1.558 Las Campanas 1987 HD 154775 1.962 1.587 7.589 0.865 1.744 Udalski and Geyer (1984) 2.006 1.532 7.616 0.874 1.742 Las Campanas 1986 2.056 1.555 7.615 0.865 1.734 Las Campanas 1987 Table II UBVRcIc OBSERVATIONS OF HD 155555 JD(hel) Phase U - B B - V V V - Rc V - Ic 2446000+ 582.590 0.590 0.303 0.776 6.728 0.451 0.910 582.709 0.662 0.302 0.783 6.749 0.452 0.909 583.608 0.204 0.265 0.805 6.773 0.451 0.897 583.705 0.262 0.281 0.795 6.757 0.451 0.897 584.625 0.816 0.286 0.786 6.774 0.450 0.898 584.738 0.884 0.265 0.782 6.761 0.452 0.903 585.607 0.408 0.298 0.786 6.717 0.448 0.885 587.659 0.644 0.287 0.789 6.725 0.443 0.886 588.620 0.223 0.329 0.790 6.798 0.457 0.906 589.626 0.829 0.279 0.788 6.744 0.444 0.885 860.878 0.234 0.317 0.805 6.801 0.457 0.916 861.852 0.820 0.338 0.799 6.729 0.446 0.904 865.865 0.238 0.274: 0.779 6.774 0.446 0.904 867.860 0.440 0.304: 0.812 6.786 0.443 0.908 868.847 0.034 0.362: 0.785 6.708 0.446 0.892 869.828 0.625 0.329 0.806 6.786 0.460 0.917 870.823 0.225 0.270: 0.799 6.755 0.445 0.899 [FIGURE 1] The mean brightness of HD 155555 during both seasons was about V = 6.76, i.e. by about 0.09 fainter than observed by Udalski and Geyer (1984) and by about 0.03 fainter than observed by Bopp et al., (1986), but in a good agreement with the somewhat lower levels observed by Lloyd Evans and Koen (1987) and Collier Cameron (1987). The amplitude DeltaV ~/= 0.08 was similar to that observed by all previous observes. The mean colour data also agree very well with the previous results (notice, however, that V - Ic of Udalski and Geyer deviates by about 0.05 from all other results). The very small colour variations of HD 155555 seem to reveal a relationship which seems to be standard for spotted stars in that most of the colours (except U - B) become redder for the decreased brightness, This tendency is better visible in B-V and V -Ic. The V -Rc colour does not seem to change by much whereas the U - B colour becomes more ultraviolet for the decreased brightness of the star. This phenomenon is also characteristic for spotted stars where dark regions are apparently surrounded by areas of increased chromospheric activity. This work has been supported by an operating grant of the Natural Sciences and Engineering Research Council of Canada. S. M. RUCINSKI David Dunlap Observatory University of Toronto P. O. Box 360 Richmond Hill, Ontario L4C 4Y6 Canada References: Bennett, N. W., Evans, D. S., and Laing, F. D.: 1962, Roy. Obs. Bull. No. 61. [BIBCODE 1962RGOB...61...91B ] Bopp, B. W., Africano, J., and Quigley, R.: 1986, Astron.J., 92, 1409. [BIBCODE 1986AJ.....92.1409B ] Collier Cameron A.: 1987, S. Afr. Astr. Obs. Circ. No. 11, 57. [BIBCODE 1987SAAOC..11...57C ] Fernandez-Figueroa, M. J., Sedano, J. L., and de Castro, E.: 1986, Astron. Astrophys., 169, 237. [BIBCODE 1986A&A...169..237F ] Lloyd Evans, T., and Koen, M. C. J.: 1987, S. Afr. Astr. Obs. Circ. No. 11, 21. [BIBCODE 1987SAAOC..11...21L ] Stacy, J. G., Stencel, R. E., and Weiler, E. J.: 1980, Astron.J., 85, 858. [BIBCODE 1980AJ.....85..858S ] Udalski, A., and Geyer, E. H.: 1984, Inf. Bull. Var. Stars, No. 2593.