COMMISSION 27 OF THE I.A.U. INFORMATION BULLETIN ON VARIABLE STARS Number 1230 Konkoly Observatory Budapest 1977 January 19 uvby PHOTOMETRY OF THE SUSPECTED VARIABLE Omega2 Sco The star omega2 Sco (HR 5997, V=4.31) is classified as a suspected variable of spectral type gG2 in the Catalogue of Bright Stars (Hoffleit 1964). The star was listed by Crawford and Barnes (1970) as a standard star in the Stromgren system, and recently Gronbech et al. (1976) used omega2 Sco as a standard for ubvy photometry of bright O to G0 stars south of declination +10deg. The author used this star as a standard star for four-colour observations at La Silla, Chile in 1973. In view of its suspected character, however, measurements of the nearby bright stars nu Sco (HR 6026-27, A0IV-B2IV, V=4.00) and omega1 Sco (HR 5993, B1V, V=3.95) were made immediately before and after each measurement of omega2 Sco. Although the spectral types of the suspected variable and the comparison stars differ more than one might like, all stars are very bright and one may expect high-precision differential photometry. The measurements were corrected for atmospheric extinction and the indices b-y, m1 and c1 were transformed to the standard system of Crawford and Barnes (1970). The y observations were transformed to the standard V magnitudes of the UBV system. Table 1 gives the mean values of the non-differential nightly means of b-y, m1 and c1 calculated from measurements obtained during 7 nights in 1973. The mean error sigma on one nightly mean value is given. The Table also contains the results of Gronbech et al. (1976) and those of Crawford and Barnes (1970). Table 1 nu Sco=HR 6026-27 omega1 Sco=HR 5993 omega2 Sco=HR 5997 b-y m1 c1 b-y m1 c1 b-y m1 c1 m 0.078 0.056 0.140 0.036 0.048 0.005 0.516 0.298 0.419 This sigma 0.001 0.004 0.004 0.001 0.004 0.005 0.004 0.006 0.007 work m 0.083 0.047 0.142 0.044 0.037 0.008 0.499 0.329 0.407 Gronbech sigma 0.003 0.004 0.004 0.003 0.004 0.003 0.003 0.004 0.004 et al. 1976 m 0.072 0.059 0.150 0.033 0.041 0.022 0.521 0.284 0.448 Crawford et al. 1970 Table 2 gives the mean values of the differential results for omega2 Sco relatively to nu Sco and also for nu Sco relatively to omega1 Sco. Table 2 omega2 Sco - nu Sco nu Sco - omega1 Sco y b-y m1 c1 y b-y m1 c1 m 0.312 0.438 0.242 0.279 0.052 0.043 0.007 0.135 sigma 0.004 0.004 0.009 0.010 0.003 0.001 0.002 0.001 The standard deviations in y for the differences omega2 Sco - nu Sco and nu Sco - omega1 Sco are similar, but the mean errors in b-y, and especially in m1 and c1 are significantly different for both groups. Figure 1 gives a plot of the differential nightly mean values y, b-y, m1 and c1. The Figure clearly indicates that omega2 Sco shows significant variations in b-y, m1 and c1, while more measurements are needed to conclude whether the y-fluctuations are significant or not. Further observations are planned. Acknowledgement The author wishes to express his thanks to the University of Copenhagen for the use of the 50 cm Danish Telescope and photometric equipment. Part of this research was supported by the National Foundation of Collective Fundamental Research of Belgium (FKFO) under contract no 2.9002.76. [FIGURE 1] C. STERKEN Astrophysical Institute, Vrije Universiteit Brussel, Belgium and European Southern Observatory, Santiago, Chile. References: Crawford, D.L. and Barnes, J.V., 1970, Astron. J. 75, 978 [BIBCODE 1970AJ.....75..978C ] Gronbech, B., Olsen, E.H. and Stromgren, B. 1976. Astron. Astrophys. Suppl. 26, 155 [BIBCODE 1976A&AS...26..155G ] Hoffleit, D. 1964 Catalogue of Bright Stars, Yale University Observatory [BIBCODE 1964cbs..book.....H ]