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The 0.4-Msun eclipsing binary CU Cancri. Absolute dimensions, comparison with evolutionary models and possible evidence for a circumstellar dust disk
Photometric observations in the R and I bands of the detached M-typedouble-lined eclipsing binary CU Cnc have beenacquired and analysed. The photometric elements obtained from theanalysis of the light curves have been combined with an existingspectroscopic solution to yield high-precision (errors la 2%) absolutedimensions: MA=0.4333+/-0.0017 Msun,MB= 0.3980+/-0.0014 Msun,RA=0.4317+/-0.0052 Rsun, andRB=0.3908+/-0.0094 Rsun. The mean effectivetemperature of the system has been estimated to be Teff=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 ( ~320 Myr) and chemical composition ([Fe/H]~ 0.0) of the binary systemthrough its membership of the Castor moving group. With all theseobservational constraints, we have carried out a critical test of recentstellar models for low-mass stars. The comparison reveals that mostevolutionary models underestimate the radius of the stars by as much as10%, thus confirming the trend observed by Torres & Ribas(\cite{TR02}) for YY Gem and V818Tau. In the mass-absolute magnitude diagram, CU Cnc isobserved to be dimmer than other stars of the same mass and this makesthe comparison with stellar models not so compelling. After ruling out anumber of different scenarios, the apparent faintness of CU Cnc can beexplained if its components are some 10% cooler than similar-mass starsor if there is some source of circumstellar dust absorption. The lattercould be a tantalizing indirect evidence for a coplanar (Vega-like)dusty disk around this relatively young M-type binary.Tables 1 and 2 are only available in electronic form at the CDS viaanonymous ftp to cdsarc.u-strasbg.fr (130.79.125.5) or viahttp://cdsweb.u-strasbg.fr/cgi-bin/qcat?J/A+A/398/239}
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