5 Conclusions

Our survey of photometric variability among 990 stars in the old open cluster M 67 detected 69 variable stars. Among the brightest sources in our sample, detection of variability $\sigma\approx 10$ mmag (with >$3\sigma$ confidence) is achieved; for the typical star observed, sensitivity to variability at levels $\sigma\approx 20$ mmag is achieved. Membership information is available for 439 stars (46 variables) included in our observations and marks 319 (38 variables) as members with a probability of at least 75% (Sanders 1977; Girard et al. 1989). Of these 38 variable cluster members, 29 exhibit variability in more than one of the passbands used, increasing our confidence that the observed variability is real. Nine of these stars are periodic variables.

In all cases the amplitude of variability is low, ranging from a few hundredths to a few tenths of a magnitude. Our study is sensitive to brightness variations on time scales of 0.3 hours to $\sim$20 days. Apparently, at the age of M 67 variability on these time scales and at these amplitudes is strongly associated with binarity, as 14 of the 29 "high-confidence" variable members are known binaries. One of the other high-confidence variables (S 1112) is an X-ray source for which binarity has not been established, another is a blue straggler (S 1263), and still two others are situated on the cluster binary sequence. Periodic variability is especially rare for single stars, for in 8 of the 9 periodic variable members observed by us (including the new candidate contact system S 757), 7 of which are X-ray sources, the variability finds its origin in the binary nature of the stars (eclipses, ellipsoidal variations, rotational spot-modulation in tidally locked binaries). This confirms the picture that rapid rotation in an old population can only be maintained in close binaries.

In the contact binary 3665 (ET Cnc), for which no membership information exists, the variability is the result of binarity as well. We encourage spectroscopic observations of the three remaining stars that exhibit periodic variations (the faint and blue star 2426 and star 2703 on the binary sequence, both without membership information) and the member 3579 (S 1112, discussed in Paper I) to establish their binary status and/or obtain an indication for membership from their radial velocity.

Also, more observations should be obtained of the stars for which we provide tentative periods, in the first place to further examine if their photometric variability is indeed periodic and secondly to establish if they are single or binary.

The origin of the photometric variability for the remaining stars discussed in this paper is in most cases unknown. As possible causes for the variations we suggest low-level surface activity, stellar pulsations or, especially for the stars on the binary sequence, binary interaction.

Acknowledgements

The authors wish to thank Magiel Janson, Rien Dijkstra, Gertie Geertsema, Remon Cornelisse and Gijs Nelemans for obtaining part of the data used in the paper. The Kitt Peak National Observatory is part of the National Optical Astronomy Observatories, which is operated by the Association of Universities for Research in Astronomy, Inc. (AURA) under cooperative agreement with the National Science Foundation. The Jacobus Kapteyn Telescope is operated on the island of La Palma by the Isaac Newton Group in the Spanish Observatorio del Roque de los Muchachos of the Instituto de Astrofisica de Canarias. The Dutch 0.91-m Telescope was operated at La Silla by the European Southern Observatory. Support for this work was provided by NASA through Hubble Fellowship grant HST-HF-01144.01-A by the Space Telescope Science Institute, which is operated by AURA, for NASA, under contract NAS 5-26555. This research made extensive use of the WEBDA Open Cluster Database developed and maintained by J.-C. Mermilliod. MvdB is supported by the Netherlands Organization for Scientific Research (NWO).