COMMISSION 27 OF THE I.A.U. INFORMATION BULLETIN ON VARIABLE STARS Number 1293 Konkoly Observatory Budapest 1977 June 15 A PHOTOELECTRIC SEARCH FOR OPTICAL VARIABILITY IN BLUE STRAGGLERS In several of the oldest galactic clusters, a few members lie above and to the left of the main sequence turn-off on the color-magnitude diagram. One hypothesis interprets these "blue stragglers" as the result of mass exchange in a close binary system in which the original secondary has received mass from the evolving primary and now lies beyond the turn- off (McCrea 1964), while the original primary has become a cool giant or sub-giant or a white dwarf. Strom and Strom (1970) have found support for this hypothesis in their photometric and radial velocity observations of blue stragglers in NGC 7789, from which they conclude that "all blue stragglers studied are probably spectroscopic binaries." In order to test the binary hypothesis we initiated a photometric program to search for periodic low-amplitude light variability of blue stragglers. Candidates were chosen by searching published color-magnitude diagrams, principally compilations by Hoag et al. (1961) and Hagan (1970). Proper motion data indicating cluster membership were available for all objects observed except NGC 6633-H159, NGC 6834-F121 and NGC 559-L85. Sixteen objects were monitored between June 1976 and April 1977, and are listed in Table I. The 38-cm siderostat refractor equipped with the Pierce-Blitzstein simultaneous dual channel, pulse-counting photometer and the 72-cm reflector of the Flower and Cook Observatory were used. Each comparison star was checked nightly against a third star to assure its constancy. The fainter objects were observed without color filtering. Table I includes results of our observations. Each object is listed according to its familiar designation within the cluster, along with other identifications. The values of V, B-V, and spectral type are taken from Mermilliod (1976), Funfschilling (1967) and Cudworth (1976). The ninth, tenth, and eleventh columns contain the number of nights, filters used, and the total number of filtered or unfiltered observations. The standard deviation of an individual magnitude difference from the mean (sigma), and the standard deviation of an individual magnitude difference calculated from Poisson statistics (sigma1) are listed for each object in the last two columns. The value of sigma1 depends in part on the integration time and the telescope aperture, and shows consistency when reduced to a common scale. No periodic variations have been identified in any of these objects. The following stars have, however, shown some variability. Coma-T146 (HR 4752, AI Com, 17 Com) has been found by Preston, Stepien, and Wolff (1969) to show periodic magnetic and light variations. A similarly low-amplitude variation is observed in our data, but it would require a shift in phase to be consistent with their value for the period. NGC 752-Hm209 may be an object similar to AI Com, and variations of 0.05 in B have been observed. A secular variation was observed in the magnitude difference between NGC 6633-H66 and its comparison star. Over about 80 days, H66 decreased 0.08m and 0.05 in B and V, respectively. The observations of NGC 6834-F121 show four minima of 0.15 depth in B. Two minima and one maximum, with a light variation of about 0.1m appear to be present in the data for NGC 7062-J155. M67-F190 deserves special note. Deutsch (1968) found it to be a spectroscopic binary with a period of 4.198 days. We were unable to find a light variation corresponding to this period. From its position in the color-magnitude diagram, Eggen (1971) indicated that it might be a delta Sct-type star. Our observations rule out any magnitude variation greater than 0.02m during this interval. NGC 6633-H159 was observed by Hintzen, Scott, and Whelan (1974) and noted as a possible velocity variable. Our limited coverage indicates some variation. Perhaps further observations might show it to be both a velocity and light variable. However, the cluster membership of this star is uncertain. Although not ruling out the binary mass exchange hypothesis, the absence of periodic light variations in these observations does set restrictions on the values of the periods and inclinations possible for these systems. We intend to continue monitoring some of these objects. BRUCE J. HRIVNAK Department of Astronomy University of Pennsylvania Philadelphia, PA USA References: Cudworth, K.M. 1976, Astron. Astrophys. Suppl. 24, 143. [BIBCODE 1976A&AS...24..143C ] Deutsch, A.J. 1968, quoted in Annual Report, Mount Wilson and Palomar Observatories, 1967-68, p. 24. Eggen, O.J. 1971, Pub. Astron.Soc. Pac. 83, 762. [BIBCODE 1971PASP...83..762E ] Funfschilling, H. 1967, Z. Astrophys. 66, 440. [BIBCODE 1967ZA.....66..440F ] Hagan, G.L. 1970, Pub. David Dunlap Obs. 4. [BIBCODE 1970PDDO....4....1H ] Hintzen, P., Scott, J., and Whelan, J. 1974, Astrophys. J. 194, 657. [BIBCODE 1974ApJ...194..657H ] Hoag, A.A., Johnson, H.L., Iriarte, B., Mitchell, R.J., Hallam, K.L., and Sharpless, S. 1961, Pub. U.S. Naval Obs., 17, part 7. [BIBCODE 1961PUSNO..17....1H ] McCrea, W.H. 1964, Mon. Not. R. Astron. Soc. 128, 147. [BIBCODE 1964MNRAS.128..147M ] Mermilliad, J.-C. 1976, Astron. Astrophys. Suppl. 24, 143. [BIBCODE 1976A&AS...24..159M ] Preston, G.W., Stepien, K., and Wolff, S.C. 1969, Astrophys. J. 156, 653. [BIBCODE 1969ApJ...156..653P ] Strom, K.M., and Strom, S.E. 1970, Astrophys. J. 162, 523. [BIBCODE 1970ApJ...162..523S ] TABLE I Summary of Blue Straggler Monitoring Program Cluster Object HD BD V B-V Sp N Filter n sigma sigm1 NGC 559 L85 10.60 +.36 9 none 113 +-.055 +-.011 NGC 752 Hm209 +36d0367 9.70 +.05 B9.5(p?) 8 V 108 .012 .006 B 110 .024 .005 M 67 F81 10.03 -.073 B8 V 7 none 37 .017 .005 F124 12.14 +.45 4 none 11 .061 .011 F131 11.22 +.415 F0 IV 5 none 29 .028 .007 F136 11.31 +.63 G3 III-IV 5 none 23 .020 .007 F153 11.31 +.13 Am 6 none 30 .022 .008 F156 10.99 +.11 A2 V 5 none 25 .018 .006 F190 10.98 +.245 A8 IV-V 15 none 79 .013 .006 III-12 12.27 +.27 5 none 22 .137 .012 Coma T146 +26d2354 5.23 -.049 A0p 4 V 188 .011 .005 B 203 .011 .004 NGC 6633 H32 169959 +6d3762 7.57 +.09 20 V 351 .023 .008 B 360 .026 H66 170054 +6d3772 8.18 +.03 19 V 280 .033 .008 B 270 .040 .007 H153 170563 +6d3816 8.15 -.15 6 V 64 .024 .010 B 67 .037 .009 NGC 6834 F121 11.14 +.48 20 V 171 .074 .015 B 180 .077 .013 NGC 7062 J155 +11.87 +.50 16 none 200 +-.062 +-.016