These stars, called 'Brown Dwarfs' are pretty hard to find unless they are orbiting nearby stars. But astronomers desperately want to know more about them because they are the missing links between ordinary stars like our Sun, and big planets like Jupiter.
Near the Sun, we know of the following stars which have extremely low masses, here a mass/Sun of 1.0 = the mass of the Sun and mass/Jupiter = 1 is the mass of jupiter:
Star...........Mass/Sun........Mass/Jupiter Ross 614B 0.080 84 Gliese 623B 0.080 84 Gliese 165B 0.080 84 Wolf 424A 0.060 63 Wolf 424B 0.050 52 L726-8 0.044 46 UV Ceti 0.035 37For comparison, Jupiter has a mass of 0.00096 times the Sun.
Just recently, astronomers at Caltech and Johns Hopkins University reported discovering a faint companion to the star Gliese 229 in the constellation Lepus. See the January 1996 issue of Sky and Telescope for more details. This companion, still under study to pinpoint its mass, has been estimated as being only 20 - 50 times the mass of Jupiter. At these masses, brown dwarfs have temperatures of only 1500 K, and their masses are 2 times too small for nuclear fusion to be ignited very efficiently in their cores. Hubble Space Telescope views of the brown dwarf candidates Gliese 229B and Gliese 105B are also available.