Abstract. In this article we present the case of HD 41004 AB, a system composed of a K0V star and a 3.7-mag fainter M-dwarf companion. We have obtained 86 CORALIE spectra of this system with the goal of obtaining precise radial-velocity measurements. Since HD 41004 A and B are separated by only 0.5 , in every spectrum taken for the radial-velocity measurement, we are observing the blended spectra of the two stars. An analysis of the measurements has revealed a velocity variation with an amplitude of about 50 m s −1 and a periodicity of 1.3 days. This radial-velocity signal is consistent with the expected variation induced by the presence of a companion to either HD 41004 A or HD 41004 B, or to some other effect due to e.g. activity related phenomena. In particular, such a small velocity amplitude could be the signature of the presence of a very low mass giant planetary companion to HD 41004 A, whose light dominates the spectra. The radial-velocity measurements were then complemented with a photometric campaign and with the analysis of the bisector of the CORALIE Cross-Correlation Function (CCF). While the former revealed no significant variations within the observational precision of ∼0.003-0.004 mag (except for an observed flare event), the bisector analysis showed that the line profiles are varying in phase with the radial-velocity. This latter result, complemented with a series of simulations, has shown that we can explain the observations by considering that HD 41004 B has a brown-dwarf companion orbiting with the observed 1.3-day period. As the spectrum of the fainter HD 41004 B "moves" relative to the one of HD 41004 A (with an amplitude of a few km s −1 ), the relative position of the spectral lines of the two spectra changes, thus changing the blended line-profiles. This variation is large enough to explain the observed radial-velocity and bisector variations, and is compatible with the absence of any photometric signal. If confirmed, this detection represents the first discovery of a brown dwarf in a very short period (1.3-day) orbit around an M dwarf. Finally, this case should be taken as a serious warning about the importance of analyzing the bisector when looking for planets using radial-velocity techniques.

Context. Models of stellar structure and evolution can be constrained by measuring accurate parameters of detached eclipsing binaries in open clusters. Multiple binary stars provide the means to determine helium abundances in these old stellar systems, and in turn, to improve age estimates. Aims. Earlier measurements of the masses and radii of the detached eclipsing binary V20 in the open cluster NGC 6791 were accurate enough to demonstrate that there are significant differences between current stellar models. Here we improve on those results and add measurements of two additional detached eclipsing binaries, the cluster members V18 and V80. The enlarged sample sets much tighter constraints on the properties of stellar models than has hitherto been possible, thereby improving both the accuracy and precision of the cluster age. Methods. We employed (i) high-resolution UVES spectroscopy of V18, V20 and V80 to determine their spectroscopic effective temperatures, [Fe/H] values, and spectroscopic orbital elements; and (ii) time-series photometry from the Nordic Optical Telescope to obtain the photometric elements. Results. The masses and radii of the V18 and V20 components are found to high accuracy, with errors on the masses in the range 0.27-0.36% and errors on the radii in the range 0.61-0.92%. V80 is found to be magnetically active, and more observations are needed to determine its parameters accurately. The metallicity of NGC 6791 is measured from disentangled spectra of the binaries and a few single stars to be [Fe/H] = +0.29 ± 0.03 (random) ± 0.07 (systematic). The cluster reddening and apparent distance modulus are found to be E(B − V) = 0.160 ± 0.025 and (m − M) V = 13.51 ± 0.06. A first model comparison shows that we can constrain the helium content of the NGC 6791 stars, and thus reach a more accurate age than previously possible. It may be possible to constrain additional parameters, in particular the C, N, and O abundances. This will be investigated in Paper II. Conclusions. Using multiple, detached eclipsing binaries for determining stellar cluster ages, it is now possible to constrain parameters of stellar models, notably the helium content, which were previously out of reach. By observing a suitable number of detached eclipsing binaries in several open clusters, it will be possible to calibrate the age-scale and the helium enrichment parameter ΔY/ΔZ, and provide firm constraints that stellar models must reproduce.

Context. We wish to determine accurate ages for open clusters and use this, in conjunction with colour-magnitude diagrams, to constrain models of stellar structure and evolution. Aims. The detached eclipsing binary V20 in the old, metal-rich ([Fe/H] = +0.40) open cluster NGC 6791 is studied in order to determine highly accurate masses and radii of its components. This allows the cluster age to be established with high precision, using isochrones in the mass-radius diagram. Methods. We employ high-resolution UVES spectroscopy of V20 to determine the spectroscopic orbit and time-series V, I photometry to obtain the photometric elements. Results. The masses and radii of the V20 components are found to be 1.074 ± 0.008 M and 1.399 ± 0.016 R (primary) and 0.827 ± 0.004 M and 0.768 ± 0.006 R (secondary). The primary is located almost exactly at the hottest point along the cluster isochrone, and the secondary is a ∼7 times fainter main-sequence star. We determine an apparent cluster distance-modulus of (m − M) V = 13.46 ± 0.10 (average of primary and secondary). The cluster age is obtained from comparisons with theoretical isochrones in the mass-radius diagram. Using the isochrones from Victoria-Regina with [Fe/H] = +0.37 we find 7.7 ± 0.5 Gyr, whereas the Yonsei-Yale (Y 2 ) isochrones lead to 8.2 ± 0.5 Gyr, and BaSTI isochrones to 9.0 ± 0.5 Gyr. In a mass-radius diagram, the 7.7 Gyr VRSS and 9.0 Gyr BaSTI isochrones overlap nearly perfectly despite the age-difference. This model dependence, which is significantly larger than the precision determined from mass, radius, and abundance uncertainties, prevents a definitive age-determination of the cluster. Conclusions. Using detached eclipsing binaries for determination of cluster ages, the dominant error is due to differences among stellar models and no longer to observational errors in cluster reddening and distance. By observing a suitable number of detached eclipsing binaries in several open clusters it should be possible to calibrate the age-scale and provide firm constraints which stellar models must reproduce.

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We present time series radial velocity, and photometric observations of a solar-type double-lined eclipsing binary star (V 12) in the old open cluster NGC 188. We use these data to determine the spectroscopic orbit and the photometric elements for V 12. From our analysis, we determine accurate masses (M p = 1.103 ± 0.007 M , M s = 1.081 ± 0.007 M ) and radii (R p = 1.424 ± 0.019 R , R s = 1.373 ± 0.019 R ) for the primary (p) and secondary (s) binary components. We adopt a reddening of E B−V = 0.087 for NGC 188, and derive component effective temperatures of 5900 ± 100 K and 5875 ± 100 K, respectively, for the primary and secondary stars. From their absolute dimensions, the two components of V 12 yield identical distance moduli of V 0 − M V = 11. m 24 ± 0. m 09, corresponding to 1770 ± 75 pc. Both stars are near the end of their main-sequence evolutionary phase, and are located at the cluster turnoff in the color-magnitude diagram. We determine an age of 6.2 ± 0.2 Gyr for V 12 and NGC 188, from a comparison with theoretical isochrones in the mass-radius diagram. This age is independent of distance, reddening, and color-temperature transformations. We use isochrones from Victoria-Regina (VRSS) and Yonsei-Yale (Y 2 ) with [Fe/H] = −0.1 and [Fe/H] = 0.0. From the solar metallicity isochrones, an age of 6.4 Gyr provides the best fit to the binary components for both sets of models. For the isochrones with [Fe/H] = −0.1, ages of 6.0 Gyr and 5.9 Gyr provide the best fits for the (VRSS) and (Y 2 ) models, respectively. We use the distance and age estimates for V 12, together with best estimates for the metallicity and reddening of NGC 188, to investigate the locations of the corresponding VRSS and Y 2 isochrones relative to cluster members in the color-magnitude diagram. Plausible changes in the model metallicity and distance to better match the isochrones to the cluster sequences, result in a range of ages for NGC 188 that is more than 3 times that resulting from our analysis of V 12.