The structure and properties of the diffuse interstellar medium (ISM) on tiny scales (sub-au to 1 pc) are poorly understood. We compared interstellar absorption lines, observed towards a selection of O- and B-type stars at two or more epochs, to search for variations over time caused by the transverse motion of each star combined with changes in the structure in the foreground ISM. Two sets of high resolution, high signal-to-noise ratio data were used: 83 VLT/UVES spectra with approximately 6 years between epochs and 21 McDonald observatory 2.7m telescope echelle spectra with 6 – 20 years between epochs, over a range of scales from 2 – 360 au. Of the 104 sightlines investigated, possible temporal variations were identified in 5 UVES spectra, in Ca II, Ca I and/or Na I absorption lines. The variations detected range from 7% to a factor of 16 in column density. No variation was found in any other interstellar species. Most sightlines show no variation, with 3-sigma upper limits to changes of the order 0.1 to 0.3 dex in CaII and NaI.
These variations observed imply that fine-scale structure is present in the ISM, but at the resolution available in this study, is not very common at visible wavelengths. A determination of the electron densities and lower limits to total number density of a sample of the sightlines implies that there is no striking difference between these parameters in sightlines with, and sightlines without, varying components.