New neighbours - I. 13 new companions to nearby M dwarfs

We present preliminary results of a long-term radial-velocity search for companions to nearby M dwarfs, started in September 95. The observed sample is volume-limited, and defined by the 127 northern (delta > -16 degrees) M dwarfs listed in the Gliese and Jahreiss (CNS3) catalogue with d less than or equal to 9 pc and V less than or equal to 15. Observations are obtained with the ELODIE spectrograph on the 1.93-m telescope of the Observatoire de Haute-Provence. The typical accuracy ranges between 10 m s(-1) (the instrumental stability limit) for the brighter stars and 70 m s(-1) at our limiting magnitude. We complement the ELODIE velocities with older measurements extracted from the CORAVEL database to extend our time base, albeit obviously with lower precision. Simultaneously, we perform adaptive optics imaging at CFHT and ESO to look for close (a >0.05-0.1 ") visual companions in a larger volume-limited sample. For stellar companions the two techniques together cover the full separation range, to beyond the limiting distance of the sample. We will therefore eventually obtain a statistically meaningful inventory of the stellar multiplicity of nearby M-dwarf systems. We also have useful sensitivity to giant planets, as illustrated by our recent detection of a planetary companion to G1 876. After 2.5 years, we have discovered 12 previously unknown components in this 127 stars sample, plus a companion to an additional star beyond its southern declination limit. 7 of these are actually beyond the 9 pc limit, as they belong to systems included in the sample on the basis of CNS3 photometric parallaxes which were biased-down by the unrecognized companion. The remaining 5 companions are true additions to the 9 pc inventory. More are certainly forthcoming, given our present selection bias towards short periods and relatively massive companions. We have derived orbital elements for 7 of the new systems, as well as for some known binaries with previously undetermined orbits. One system, G 203-47, associates an M3.5V star with a white dwarf in a rather tight orbit (a(1) sin i = 15 R.) and represents a Post-Common-Envelope system. Some of the new M-dwarf binaries will over the next few years provide very precise mass determinations, and will thus better constrain the still poorly determined lower main-sequence mass-luminosity relation. The first such results are now being obtained, with some preliminary accuracies that range between 2% at 0.4-0.6 M. and 10% at 0.1 M.. We have also discovered the third known detached M-dwarf eclipsing binary, and determined its masses with 0.4% accuracy.