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The structure and properties of the diffuse interstellar medium (ISM) onsmall scales, sub-au to 1 pc, are poorly understood. We compare interstellarabsorption-lines, observed towards a selection of O- and B-type stars at two ormore epochs, to search for variations over time caused by the transverse motionof each star combined with changes in the structure in the foreground ISM. Twosets of data were used: 83 VLT- UVES spectra with approximately 6 yr betweenepochs and 21 McDonald observatory 2.7m telescope echelle spectra with 6 - 20yr between epochs, over a range of scales from 0 - 360 au. The interstellarabsorption-lines observed at the two epochs were subtracted and searched forany residuals due to changes in the foreground ISM. Of the 104 sightlinesinvestigated with typically five or more components in Na I D, possibletemporal variation was identified in five UVES spectra (six components), in CaII, Ca I and/or Na I absorption-lines. The variations detected range from 7\%to a factor of 3.6 in column density. No variation was found in any otherinterstellar species. Most sightlines show no variation, with 3{\sigma} upperlimits to changes of the order 0.1 - 0.3 dex in Ca II and Na I. Thesevariations observed imply that fine-scale structure is present in the ISM, butat the resolution available in this study, is not very common at visiblewavelengths. A determination of the electron densities and lower limits to thetotal number density of a sample of the sightlines implies that there is nostriking difference between these parameters in sightlines with, and sightlineswithout, varying components.

Early-type stars observed in the ESO UVES Paranal Observatory Project - V. Time-variable interstellar absorption