1997A&A...322..291E

Monitoring of the H₂O maser in OH 39.7+1.5 as well as spatial interferometry show clear evidence that the maser switches occasionally its beaming direction between radial and tangential paths in step with the stellar pulsation cycle. Outside the minimum, several radially beamed maser features are present, which are absent (≤20mJy) during the minimum. Competition between gain paths causes tangentially beamed maser emission only to occur, when the temperature in the maser shell is too low to excite radially beamed masers. The radius of the H₂O maser shell extends between 6x10¹⁴cm (40AU) and at least 3x10¹⁵cm (200AU), with the inner radius probably set by quenching of the maser due to high gas density close to the star. Within the H₂O maser shell, the circumstellar outflow accelerates from v≃9km/s up to the terminal expansion velocity of ≃18km/s. The expansion velocity of the circumstellar shell is constant between the outer radius of the water maser shell and the location of the 1612MHz OH maser shell at 3x10¹⁶cm, showing the absence of strong velocity gradients at large radial distances.