The 0.4-M-circle dot eclipsing binary CU Cancri - Absolute dimensions, comparison with evolutionary models and possible evidence for a circumstellar dust disk

Photometric observations in the R and 1 bands of the detached M-type double-lined eclipsing binary CU Cnc have been acquired and analysed. The photometric elements obtained from the analysis of the light curves have been combined with an existing spectroscopic solution to yield high-precision (errors less than or similar to2%) absolute dimensions: M-A = 0.4333 +/- 0.0017 M-., M-B = 0.3980 +/- 0.0014 M-., R-A = 0.4317 +/- 0.0052 R-., and R-B = 0.3908 +/- 0.0094 R-.. The mean effective temperature of the system has been estimated to be T-eff = 3140 +/- 150 K by comparing multi-band photometry (optical and infrared) with synthetic colors computed from state-of-the-art model atmospheres. Additionally, we have been able to obtain an estimate for the age (similar to320 Myr) and chemical composition ([Fe/H] approximate to 0.0) of the binary system through its membership of the Castor moving group. With all these observational constraints, we have carried out a critical test of recent stellar models for low-mass stars. The comparison reveals that most evolutionary models underestimate the radius of the stars by as much as 10%, thus confirming the trend observed by Torres & Ribas (2002) for YY Gem and V818 Tau. In the mass-absolute magnitude diagram, CU Cnc is observed to be dimmer than other stars of the same mass and this makes the comparison with stellar models not so compelling. After ruling out a number of different scenarios, the apparent faintness of CU Cnc can be explained if its components are some 10% cooler than similar-mass stars or if there is some source of circumstellar dust absorption. The latter could be a tantalizing indirect evidence for a coplanar (Vega-like) dusty disk around this relatively young M-type binary.