COMMISSION 27 OF THE I. A. U. INFORMATION BULLETIN ON VARIABLE STARS Number 3594 Konkoly Observatory Budapest 25 April 1991 HU ISSN 0374 - 0676 PHOTOMETRIC VARIABLE STARS FOUND IN FIVE SOUTHERN OPEN CLUSTERS During the years 1985 to 1989 we have been carrying out photoelectric UBV observations of the following five southern open clusters : NGC 2437, NGC 2453, NGC 2539, Cr 223 and NGC 5749. These clusters were selected for observation because they have either not been previously observed photoelectrically or the existing data are far from being complete. The results of these photometric studies are being prepared for publication. The aim of this note is to report the variability detected in twenty one stars in the region of the above mentioned clusters. Findding charts for the variable stars in NGC 2539, Cr 223 and NGC 5749 are shown in Figures 1 to 3. The new variables found in the field of NGC 2437 and NGC 2453 are shown in the finding charts published by Cuffey (1941) and Moffat and Fitzgerald (1974). A total of 567 stars were observed in the UBV system in the field of these clusters. The measurements were performed during several observing runs with the 0.6 m, 0.9 m and 1.0 m telescopes of the Cerro Tololo Inter-American Observatory (CTIO), the 0.6 m Canadian telescope of the David Dunlap Observatory located in Las Campanas Observatory (LCO) and the 2.15 m telescope of the Complejo Astronomico El Leoncito (CASLEO) located in San Juan (Argentine). Dry- ice cooled RCA 31034A and 1P21 photomultipliers were used in these observatories with pulse-counting equipments and standard UBV filters. Mean coefficients were employed in LCO and CTIO to correct for atmospheric extinction, whereas the coefficients published by Minniti et al. (1989) were used to reduce the CASLEO observations. Table 1 : Mean errors of the UBV photometry. External mean errors Epsilon_V Epsilon_BV Epsilon_UB 0.6 m (LCO) 0.009 0.006 0.011 0.6 m (CTIO) 0.012 0.009 0.009 0.9 m (CTIO) 0.011 0.005 0.013 1.0 m (CTIO) 0.008 0.009 0.010 2.15 m (CASLEO) 0.013 0.012 0.013 Internal mean errors Sigma_V Sigma_BV Sigma_UB 0.6 m (LCO) V <= 12.0 0.017 0.014 0.018 V > 12.0 0.023 0.026 0.028 0.6 m (CTIO) V <= 12.0 0.015 0.014 0.016 V > 12.0 0.018 0.022 0.026 0.9 m (CTIO) V <= 12.0 0.010 0.009 0.016 V > 12.0 0.014 0.016 0.024 1.0 m (CTIO) V <= 12.0 0.013 0.012 0.011 12.0< V <= 13.0 0.015 0.012 0.019 V > 13.0 0.022 0.022 0.023 2.15 m (CASLEO) V <= 12.0 0.016 0.017 0.019 12.0< V <= 13.0 0.021 0.019 0.022 V > 13.0 0.022 0.024 0.029 The UBV standard system was established by nightly observing between 11 and 18 standard stars selected from the lists of Cousins (1967, 1973, 1974) and Graham (1982). The external (epsilon) and internal (sigma) mean errors of the UBV photometry are summarized in Table 1. As shown in this table, the mean external errors Epsilon are all about 0.01 mag, independently of the telescope used. On the other hand, the mean internal errors Sigma_V, deduced from the night-to-night dispersion of the program stars, is about 0.02 mag, practically independent of the V-magnitude and telescope used. There exists, however, a small increase in Sigma_BV and Sigma_UB with the V-magnitude. Table 2 : Individual UBV observations of new variable stars found in southern open clusters. STAR V B-V U-B Sp. Type Membership NGC 2437 (Cuffei 1941) 174 10.640 1.110 0.882 K pm 10.727 0.964 0.869 NGC 2453 (Moffat and Fitzgerald 1979) 50 10.519 2.037 1.756 K3II/Ib m 10.740 2.003 1.200 NGC 2539 (Claria et al. 1991) 1 12.793 0.540 -0.074 ? nm 12.618 0.588 -0.195 12.630 0.615 0.044 2 13.172 0.570 -0.116 F5 pm 13.274 0.402 -0.038 3 11.804 0.166 0.153 A3/5 m 11.921 0.130 0.160 11.860 0.144 0.135 4 13.504 0.155 -0.114 ? nm 13.375 0.495 0.055 5 13.348 0.392 0.000 F2 m 13.315 0.409 0.136 13.427 0.373 0.096 6 11.648 0.909 0.399 ? nm 11.993 0.289 0.117 11.765 0.933 0.646 7 13.016 0.298 0.192 A5/7 m 12.206 0.187 0.165 8 13.393 0.366 -0.078 F0 m 13.504 0.330 0.110 9 13.580 1.301 0.919 K nm 13.749 1.332 1.416 10 12.432 0.305 0.087 F0 pm 12.948 0.431 0.026 12.971 0.333 0.099 11 12.954 0.384 0.074 F0/5 m 14.179 0.486 -0.005 Cr 223 (Claria and Lapasset 1991) 10 12.629 0.772 0.250 G5 nm 12.734 0.855 0.347 12.681 0.820 0.299 41 12.716 0.168 -0.130 B8V m 13.054 0.136 -0.087 42 12.709 1.660 1.736 G5/8 nm 12.478 0.921 0.575 105 11.683 0.105 -0.308 B6V m 11.781 0.054 -0.325 11.843 0.013 -0.357 109 13.442 0.433 0.292 A7/9 nm 13.668 0.426 0.197 NGC 5749 (Claria et al. 1991) 40 12.497 1.512 - K4III nm 12.640 1.401 1.217 64 12.972 0.444 0.276 B9V m 13.086 0.390 0.247 12.991 0.416 0.232 82 13.051 0.413 0.247 B9V pm 13.736 0.689 0.339 [FIGURE 1] Figure 1: Finding chart for the variable found in NGC 2539. [FIGURE 2] Figure 2: Finding chart for the variable found in Cr 223. [FIGURE 3] Figure 3: Finding chart for the variable found in NGC 5749. We have considered a star to be a photometric variable when its individual V measures during different nights displayed variations greater than five times the mean internal error, i.e., DeltaV > 0.1 mag. To discern whether a star is a member (m), probable member (pm) or non-member (nm), we have applied two membership criteria described by Claria and Lapasset (1986). The probable membership of the red variable stars in the field of NGC 2437 and NGC 2453 was evaluated from combined UBV and DDO (still unpublished) data by applying the criteria described by Claria and Lapasset (1983). Among the new variables detected there are thirteen which are found to be members or probable members of the studied clusters (Claria and Lapasset 1991, Claria et al. 1991), whereas the remaining eight stars are very likely field stars. Four of the new variables exhibit DeltaV variations greater than 0.50 mag, five show DeltaV values between 0.20 mag and 0.50 mag, and the remaining twelve stars have DeltaV variations in the interval 0.09 mag < DeltaV < 0.20 mag. The individual UBV measurements of the new variables are shown in Table 2. The references for star identifications are given at the head of each section of the table. Column (5) lists the spectral type as estimated from the UBV colours, excepting for stars of NGC 2437 and NGC 2453 wherein the MK spectral types were deduced from the unreddened DDO colours (Claria et al. 1991). The last column of Table 2 indicates if the star is believed to be a cluster member, probable member or non-member field star. We are very indebted to staff members and night assistants of CTIO, LCO, and CASLEO for the alloted observing runs and kind hospitality. Thanks are also due to J.E. Laborde and B. Candellero for the preparation of figures. This work was partially supported by CONICET and CONICOR of Argentine. E. LAPASSET *1, J.J. CLARIA *2, *3 Observatorio Astronomico, Universidad Nacional de Cordoba, Argentina and D. MINNITI *3 Steward Observatory, University of Arizona, USA. *1 Visiting astronomer, University of Toronto (David Dunlap Observatory) 24-inch telescope at Las Campanas Observatory. *2 Visiting astronomer of Cerro Tololo Inter-American Obseratory supported by the National Science Foundation under contract No. AST 74- 04128. *3 Visiting astronomer of Complejo Astronomico El Leoncito (CASLEO), Argentine. References Claria, J.J. and Lapasset, E. 1983, J. Astrophys. Astron. 4, 117. [BIBCODE 1983JApA....4..117C ] Claria, J.J. and Lapasset, E. 1986, Astron. J. 91, 326. [BIBCODE 1986AJ.....91..326C ] Claria, J.J. and Lapasset, E. 1991, Pub. A. S. P. (submitted). [BIBCODE 1991PASP..103..998C ] Claria, J.J., Lapasset, E. and Minniti, D. 1991 (in preparation). [IBVS 3594] Cousins, A.W.J. 1967, Mon. Not. Astron. Soc. S. Africa 26, 151. [BIBCODE 1967MNSSA..26..151C ] Cousins, A.W.J. 1973, Mem. Roy. Astron. Soc. 77, 223. [BIBCODE 1973MmRAS..77..223C ] Cousins, A.W.J. 1974, Mon. Not. Astron. Soc. S. Africa 33, 149. [BIBCODE 1974MNSSA..33..149C ] Cuffei, J., 1941, Astrophys. J. 94, 55. [BIBCODE 1941ApJ....94...55C ] Graham, J.A. 1982, Pub. A. S. P. 94, 244. [BIBCODE 1982PASP...94..244G ] Minniti, D., Claria, J.J. and Gomez, M.N., 1989, Astrophys. Space Sci. 158, 9. [BIBCODE 1989Ap&SS.158....9M ] Moffat, A.F.J. and Fitzgerald, M.P. 1974, Astron. Astrophys. suppl. 18, 19. [BIBCODE 1974A&AS...18...19M ]