P oS(IX EVN Symposium)034 Maser studies in evolved stars

High resolution maps of maser emission provide very detailed information on processes occur-ring in circumstellar envelopes of late-type stars. A particularly detailed picture of the innermost shells around AGB stars is provided by SiO masers. Considerable progress is being made to pro-vide astrometrically aligned multi-transition simultaneous observations of these masers, which are needed to better constrain the models. In view of the large amount of high quality data avail-able, models should now be developed to fully explain all maser characteristics together (spatial distribution, variability, etc). New generation instruments (VERA, VSOP-2), new observational techniques (frequency-phase transfer), and new models promise important improvements of our knowledge on this topic