Astrometric study of Lalande 21185.

Only a Centauri distant 4.3 light years, and Barnard's star distant 6.0 light years are known to be definitely nearer than Lalande 21185, Wolf 359 and L726-8. The latter three stars vie at present for third place in order of stellar distance; about 8 light years. Since the discovery of variable proper motion in 1942 by Dr. G. Land, Lalande 211 8 has been kept under observa tion with the ~-inch Sproul refractor. A recent study based on 679 plates taken on 188 nights over the interval 1937-1950 shows that the perturbation is well represented by the following dynamical elements: Period 1.14 years, eccentricity .75, periastron passage 1941.12. The geometric elements of the photometric orbit yield a seml-axls major of `(029 or .07 astronomical units, for a parallax of `(41. The corresponding value for the lower limit of the companion mass is .o66 (M $ m)O. The apparent magnitude and spectrum of La lande 21185 are 7.6 and M2, the absolute visual magnitude 10.7 and the bolometric magnitude 8.7. In view of the results for the primary, we assume .250 and .5oO as resonable lower and upper limits for the sum of the masses. In the former case, the lower limit for the companion mass 1s.030, in the latter case it is .040. If the companion is not dark, but say two magnitudes fainter than the primary, the values for the companion mass would be .060 and .110, respectively. The next maximum separation, about `( , occurs at 1952.1. The expected range in radial velocity has a minimum value of 6 km/sec. In view of the brightness of the star, such a range could probably be established. Sprout Observatory, Swarthmore College, Swarthmore, Pa.