Sub-Subgiants in the Old Open Cluster M67?

We report the discovery of two spectroscopic binaries in the field of the oldopen cluster M67 -- S1063 and S1113 -- whose positions in the color-magnitudediagram place them approximately 1 mag below the subgiant branch. A ROSAT studyof M67 independently discovered these stars to be X-ray sources. Both haveproper-motion membership probabilities greater than 97%; precise center-of-massvelocities are consistent with the cluster mean radial velocity. S1063 is alsoprojected within one core radius of the cluster center. S1063 is a single-linedbinary with a period of 18.396 days and an orbital eccentricity of 0.206. S1113is a double-lined system with a circular orbit having a period of 2.823094days. The primary stars of both binaries are subgiants. The secondary of S1113is likely a 0.9 Mo main-sequence star, which implies a 1.3 Mo primary star. Wehave been unable to explain securely the low apparent luminosities of theprimary stars; neither binary contain stars presently limited in radius bytheir Roche lobes. We speculate that S1063 and S1113 may be the products ofclose stellar encounters involving binaries in the cluster environment, and maydefine alternative stellar evolutionary tracks associated with mass-transferepisodes, mergers, and/or dynamical stellar exchanges