COMMISSION 27 OF THE I. A. U. INFORMATION BULLETIN ON VARIABLE STARS Number 3586 Konkoly Observatory Budapest 4 April 1991 HU ISSN 0374 - 0676 Photometry of two candidates for M67 contact binaries AG Cnc has been classified as a variable star by Kurochkin (1960; AG Cnc=SVS 1283). Based on 36 photometric measurements derived from photographic plates he concluded that its period of variations is equal to 0.313335 day and that the total amplitude of the light curve is about 0.6 mag. AG Cnc is located in the field of the open cluster M67. According to the results of the proper motion study, it is a highly probable member of the cluster (Sanders 1977; star No. 1113; its membership probability is 96 per cent). Kurochkin (1960) suggested that AG Cnc belongs to dwarf cepheids. However, its period and an absolute magnitude (derived under the assumption that it is a M67 member) suggest that it may be a short period eclipsing binary. M67 is known to possess at least three W UMa-type systems: AH Cnc (Kurochkin 1960; Efremov et al. 1964) and two systems discovered recently by Gilliland et al. (1991). The star IV-25 (designation according to Eggen and Sandage 1964) is located in the central region of M67 and its proper motion indicates that it is a cluster member (Sanders 1977; star No. 1063; its membership probability is 93 per cent). Racine (1971) concluded from the comparison of his photographic photometry and photoelectric photometry published by Eggen and Sandage (1964) that the brightness of IV-25 varies by at least 0.18 magnitude. On the other hand, Frolov (1983) failed to detect any evidence for a variability of IV-25 on six photographic plates. Results of the recent proper motion study of M67 by Girard et al. (1989) also support membership of AG Cnc and IV-25 to M67. They derived membership probabilities 97% and 98% for IV-25 and AG Cnc respectively. Table 1 BV photometry of AG Cnc. Phases are calculated for a period 0.31333 d and for an arbitrary ephemeris. JDhel 2446700+ B V Phase 74.044 14.849 13.782 0.142 75.987 -- 13.766 0.340 75.989 14.831 13.788 0.346 76.052 14.853 -- 0.549 76.055 14.848 13.803 0.558 78.055 14.871 13.758 0.941 78.055 14.855 0.940 78.890 14.863 -- 0.607 79.019 14.899 13.837 0.016 79.046 14.895 13.847 0.103 79.816 14.834 13.785 0.561 80.061 14.850 13.777 0.343 To clarify the questions concerning the nature of AG Cnc and IV-25 we collected a CCD photometry of these stars. Observations were performed using a #1 0.9-m telescope of the Kitt Peak National Observatory. During the period between Dec. 9 and Dec. 15 1986 we collected 11 B and 9 V frames of the 7.3x4.6 arcmin field centered on AG Cnc. The obtained photometric data are given in Table 1. The transformation to the standard BV system was based on the observations of several stars from the central part of M67. The standard error of an individual measurement is about 0.015. We phased our observations with a period of 0.313335 day and concluded that AG Cnc hardly exhibits any large amplitude variability with the quoted period. However, there is a strong evidence that on the frames collected on Dec. 14 the star was by about 0.05 mag fainter than during the remaining measurements (it is worthy to point out that this weakening was observed in both filters). The area covered by our frames contains 17 stars with B>17. None of these stars, with an exception of AG Cnc, exhibited a variability with a full amplitude exceeding 0.040 mag. Our photometry of IV-25 is given in Table 2. On the frames collected on Dec 9, the star was by about 0.10 magnitude fainter than during the remaining observations. Table 2 BV photometry of M67 star IV-25 JDhel 2446700+ B V 74.042 14.599 13.582 74.048 14.589 13.579 75.981 14.529 -- 76.056 14.531 13.518 78.054 14.503 13.498 78.891 14.504 13.523 79.022 14.520 13.499 79.047 14.491 13.493 79.8170 14.483 13.485 80.060 14.480 13.488 [FIGURE 1] Figure 1 - Color-magnitude diagram for a subset of M67 stars with the positions of AG Cnc and IV-25 indicated. In Figure 1 we show a color-magnitude diagram for a part of a central region of M67 with the marked positions of AG Cnc and IV-25. Both stars are located 2 magnitudes above the cluster main sequence in the same area of the diagram. Their colors are too red and their magnitudes are too faint as for a star from the M67 subgiant branch (for more complete color-magnitude diagram see Racine, 1971). Concluding, our photometry supports claims about variability of AG Cnc and IV-25. At the same time it rules out large amplitude variations of AG Cnc with the period of about 8 hours. The nature of variability of both stars remains unknown. The strange positions of both stars on the color magnitude diagram of M67 support the hypothesis about a binary nature of AG Cnc and IV-25. We note that, two M67 contact binaries identified recently by Gilliland et al. (1991) possess light curves whose full amplitudes do not exceed 0.15 mag. We suggest that AG Cnc and IV-25 are highly evolved W UMa-type binaries with extremely small mass ratios and consequently small amplitudes of light variations. J. Kaluzny ^1 and J. Radczynska Warsaw University Observatory Al. Ujazdowskie 4 00-478 Warszawa, Poland ^1 Visiting Observer, Kitt Peak National Observatory, National Optical Astronomy Observatories, operated by the Association of Universities for Research in Astronomy, Inc., under contract with the NSF. References: Efremov, Y.N., Khopolov, P.N., Kukarkin, B.V., and Sharov, A.S. 1964, Inf. Bull. Var. Stars., No. 75. Eggen, O.J., and Sandage, A.R., 1964, Astrophys. J., 140, 130. [BIBCODE 1964ApJ...140..130E ] Frolov, V.N. 1983, Astron. Circular USSR, No. 1280, 2. [BIBCODE 1983ATsir1280....2F ] Gilliland, R., Brown, T.M., Duncan D.K., Suntzeff, N.B., Lockwood, G.W., Thompson, D.T., Schild, R.E., Jeffrey, W.A., Penprase, B.E., 1991, Astron. J., February issue, in print. [BIBCODE 1991AJ....101..541G ] Girard, T.M., Gundy, W.M., Lopez, C.E, van Altena, W.F., 1989, Astron. J., 98, 227. [BIBCODE 1989AJ.....98..227G ] Kurochkin, N.E. 1960, Astron. Circular USSR, No. 212, 9. [BIBCODE 1960ATsir.212....9K ] Racine, R. 1971, Astrophys. J., 168, 393. [BIBCODE 1971ApJ...168..393R ] Sanders, W.L. 1977, Astron. Astrophys. Supp., 27, 89. [BIBCODE 1977A&AS...27...89S ]