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. Double-lined, detached eclipsing binaries are our main source for accurate stellar masses and radii. In this paper we focus on the 1.15-1.70 M interval where convective core overshoot is gradually ramped up in theoretical evolutionary models. Aims. We aim to determine absolute dimensions and abundances for the F-type detached eclipsing binary BK Peg, and to perform a detailed comparison with results from recent stellar evolutionary models, including a sample of previously studied systems with accurate parameters. Methods. uvby light curves and uvbyβ standard photometry were obtained with the Strömgren Automatic Telescope, ESO, La Silla, and high-resolution spectra were acquired with the FIES spectrograph at the Nordic Optical Telescope, La Palma. Results. The 5. d 49 period orbit of BK Peg is slightly eccentric (e = 0.053). The two components are quite different with masses and radii of (1.414±0.007 M , 1.988±0.008 R ) and (1.257±0.005 M , 1.474±0.017 R ), respectively. The measured rotational velocities are 16.6 ± 0.2 (primary) and 13.4 ± 0.2 (secondary) km s −1 . For the secondary component this corresponds to (pseudo)synchronous rotation, whereas the primary component seems to rotate at a slightly lower rate. We derive an iron abundance of [Fe/H] = −0.12 ± 0.07 and similar abundances for Si, Ca, Sc, Ti, Cr and Ni. The stars have evolved to the upper half of the main-sequence band. YonseiYale and Victoria-Regina evolutionary models for the observed metal abundance reproduce BK Peg at ages of 2.75 and 2.50 Gyr, respectively, but tend to predict a lower age for the more massive primary component than for the secondary. We find the same age trend for three other upper main-sequence systems in a sample of well studied eclipsing binaries with components in the 1.15-1.70 M range. We also find that the Yonsei-Yale models systematically predict higher ages than the Victoria-Regina models. The sample includes BW Aqr, and as a supplement we have determined a [Fe/H] abundance of −0.07 ± 0.11 for this late F-type binary. Conclusions. We propose to use BK Peg, BW Aqr, and other well-studied 1.15-1.70 M eclipsing binaries to fine-tune convective core overshoot, diffusion, and possibly other ingredients of modern theoretical evolutionary models.

Abstract. We present complete uvby light curves of 9 recently discovered eclipsing binaries having late F, G, and K type components within or near the main-sequence band. They are the first results from a long term program carried out since 1994 at the Strömgren Automatic Telescope at ESO, La Silla. The aim is to provide the accurate absolute dimensions for solar-type stars needed for critical tests of the corresponding theoretical models. A serious dilemma appears to be present in the comparison of predictions from current stellar models with fundamental properties of known 0.7−1.1 M eclipsing binaries (Popper 1997b, Clausen et al. 1999b). Spectroscopic observations of the 9 eclipsing binaries are either available or are being obtained, and detailed analyses of the individual systems will be published in a series of subsequent papers. The light curves were obtained as part of a systematic search for new eclipsing systems among a large sample of (mostly) spectroscopic binaries. Catalogues of the extensive uvby and β photometry from this search will be published separately.Key words. stars: fundamental parameters -stars: binaries: close -stars: binaries: eclipsing -stars: evolutionstars: late type -techniques: photometric IntroductionDetached, double-lined eclipsing binaries are the main source of accurate (1-2%) data on stellar masses and radii. Few well studied systems with solar-type components are available, and accurate masses and radii for late F-K main-sequence stars are thus scarce. Andersen (1991) includes just three stars in the 0.7−1.1 M mass range (the secondary of FL Lyr and both components of HS Aur). Selection effects (small stars of low luminosity) are responsible for rather few eclipsing binaries with late-type main-sequence components being known, and for the domination of short-period systems in the sample. In order to identify more systems, Popper (1996) undertook an impressive spectroscopic survey of known late F-K eclipsing binaries. High-quality spectroscopic orbits of 10 additional main-sequence systems have been established from this project: RT And and CG Cyg (Popper . Light curves exist for some of these binaries, and absolute dimensions for 9 more stars below 1.1 M have been published.Several of the binaries mentioned above belong to the group of short-period (P < 1 day) RS CVn systems. They have rapidly rotating components (synchronized to the orbital period) with often fairly large relative radii r = R/a. Enhanced surface activity (spots) is present, leading to quite distorted light curves and consequently less welldetermined photometric elements. Attempts to fit current stellar models to the available data show systematic differences between models and binaries, which are not yet understood (Popper 1997b;Clausen et al. 1999b).Whereas Popper (1996) searched spectroscopically for main-sequence systems among known eclipsing binaries, we have chosen to conduct a large-scale photometric search for new eclipsing cases among late-type stars known (or suspected) to be binaries. In particula...

Abstract. Recent radial-velocity measurements obtained with the CORALIE spectrograph on the 1.2-m Euler Swiss telescope at La Silla unveil the presence of a new Jovian-mass Hot Jupiter around HD 73256. The 1.85-M Jup planet moves on an extremely short-period (P = 2.5486 d), quasi-circular orbit. The best Keplerian orbital solution is presented together with an unsuccessful photometric planetary-transit search performed with the SAT Danish telescope at La Silla. Over the time span of the observations, the photometric follow-up of the candidate has nevertheless revealed a P 14-d photometric periodicity corresponding to the rotational period of the star. This variation as well as the radial-velocity jitter around the Keplerian solution are shown to be related to the fair activity level known for HD 73256.

Context. Accurate mass, radius, and abundance determinations from binaries provide important information on stellar evolution, fundamental to central fields in modern astrophysics and cosmology. Aims. Within the long-term Copenhagen Binary Project, we aim to obtain high-quality light curves and standard photometry for double-lined detached eclipsing binaries with late A, F, and G type main-sequence components, needed for the determination of accurate absolute dimensions and abundances, and for detailed comparisons with results from recent stellar evolutionary models. Methods. Between March 1985 and July 2007, we carried out photometric observations of AD Boo, HW CMa, SW CMa, V636 Cen, VZ Hya, and WZ Oph at the Strömgren Automatic Telescope at ESO, La Silla. Results. We obtained complete uvby light curves, ephemerides, and standard uvbyβ indices for all six systems. For V636 Cen and HW CMa, we present the first modern light curves, whereas for AD Boo, SW CMa, VZ Hya, and WZ Oph, they are both more accurate and more complete than earlier data. Due to a high orbital eccentricity (e = 0.50), combined with a low orbital inclination (i = 84.• 7), only one eclipse, close to periastron, occurs for HW CMa. For the two other eccentric systems, V636 Cen (e = 0.134) and SW CMa (e = 0.316), apsidal motion has been detected with periods of 5270 ± 335 and 14 900 ± 3600 years, respectively.