Aims.Our objective is to elucidate the physical process that causes the observed observed-minus-calculated (O–C) behavior in the M4.5/M4.5 binary CM Dra and to test for any evidence of a third body around the CM Dra system.

Methods.New eclipse minimum timings of CM Dra were obtained between the years 2000 and 2007. The O–C times of the system are fitted against several functions, representing different physical origins of the timing variations.

Results.Using our observational data in conjunction with published timings going back to 1977, a clear non-linearity in O–C times is apparent. An analysis using model-selection statistics gives about equal weight to a parabolic and to a sinusoidal fitting function. Attraction from a third body, either at large distance in a quasi-constant constellation across the years of observations or from a body on a shorter orbit generating periodicities in O-C times is the most likely source of the observed O-C times. The white dwarf GJ 630.1B, a proper motion companion of CM Dra, can however be rejected as the responsible third body. Also, no further evidence of the short-periodic planet candidate described by Deeg et al. (2000, A&A, 358, L5) is found, whereas other mechanisms, such as period changes from stellar winds or Applegate's mechanism can be rejected.
Conclusions.A third body, being either a few-Jupiter-mass object with a period of 18.5 ± 4.5 years or an object in the mass range of 1.5 Mjup to 0.1 with periods of hundreds to thousands of years is the most likely origin of the observed minimum timing behavior.