Issue A&A Volume 382, Number 3, February II 2002 Page(s) 888 - 898 Section Stellar clusters and associations DOI http://dx.doi.org/10.1051/0004-6361:20011521

A&A 382, 888-898 (2002)
DOI: 10.1051/0004-6361:20011521

Photometric variability in the open cluster M 67

I. Cluster members detected in X-rays

M. van den Berg¹, K. G. Stassun², F. Verbunt¹ and R. D. Mathieu<sup>2

1</sup>  Astronomical Institute, Utrecht University, PO Box 80000, 3508 TA Utrecht, The Netherlands
²  Department of Astronomy, University of Wisconsin-Madison, 475 N Charter St, Madison, WI 53706, USA

(Received 15 January 2001/ Accepted 23 October 2001)

Abstract
We study photometric variability among the optical counterparts of X-ray sources in the old open cluster M 67 . The two puzzling binaries below the giant branch are both variables: for S 1113 the photometric period is compatible with the orbital period, S 1063 either varies on a period longer than the orbital period, or does not vary periodically. For the spectroscopic binaries S 999 , S 1070 and S 1077 the photometric and orbital periods are similar. Another new periodic variable is the main-sequence star S 1112 , not known to be a binary. An increase of the photometric period in the W UMa system S 1282 ( AH Cnc ) is in agreement with a previously reported trend. Six of the eight variables we detected are binaries with orbital periods of 10 days or less and equal photometric and orbital periods. This confirms the interpretation that their X-ray emission arises in the coronae of tidally locked magnetically active stars. No variability was found for the binaries with orbital periods longer than 40 days; their X-ray emission remains to be explained.

Key words: stars: activity -- binaries: general -- stars: variables: general -- open clusters and associations: individual: M 67 -- X-rays: stars

Offprint request: M. van den Berg, vdberg@merate.mi.astro.it

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