Distance and density of short lived HI clouds at high galactic latitudes.

Very narrow interstellar absorption lines of sodium were detected in spectra of stars in the direction of a strikingly narrow and elongated H I filament, at velocities near -1.2 km/s well correlated with the H I emission. One of the stars, SAO 30359, additionally shows an optically strong interstellar absorption line at V(LSR) = -10 km/s without corresponding H I emission. Here, the results of the optical observations are presented. The size and density of a gas clump which is assumed to be responsible for this noncoincident line is derived. From the distance to the stars and the presence of absorption features, an upper distance limit of d = 240 pc is derived for this high-latitude H I filament as well as for the gas clump.