The boundary between professional and amateur astronomers gets narrower and narrower. We present several real examples, most of them published in refereed journals, of works resulting from fruitful collaborations between key amateur astronomers in Sp
ain and professional colleagues. The common denominator of these works is the search for binaries, mostly nearby, wide, common proper-motion pairs with low-mass stellar components, including some of the most fragile systems ever found.
LP 209-28 and LP 209-27 have similar proper motions as tabulated by several catalogues. Using seven astrometric epochs spanning 59 years, we confirm a common tangential velocity by measuring a constant angular separation of rho = 666.62+/-0.09 arcsec
. Accurate SDSS and 2MASS photometry indicates that they are normal dwarfs of approximate spectral types K7 V and M3 V. However, from their apparent magnitudes, both LP 209-28 and LP 209-27 are located at 200-250 pc, from where one can deduce an astonishing projected physical separation of 0.6-0.8 pc. The system Koenigstuhl 6 AB represents another world record among the least-bound systems with low-mass star components.
The Aladin sky atlas of the Virtual Observatory has shown to be a powerful and easy-handling tool for the discovery, confirmation, and characterisation of high proper-motion, multiple stellar systems of large separation in the solar vicinity. Some of
these systems have very low mass components (at the star/brown dwarf boundary) and are amongst the least bound systems found to date. With projected physical separations of up to tens of thousands astronomical units, these systems represent a challenge for theoretical scenarios of formation of very low-mass stars and brown dwarfs. Here we show preliminary results of a novel virtual search of binary systems and companions to Luyten stars with proper motions between 0.5 and 1.0 arcsec/a.
