Mode switching of the water maser in OH39.7+1.5

Monitoring of theH2Omaser inOH39.7+1.5 aswell 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 (≤ 20 mJy) during the minimum. Competition between gain paths causes tangentially beamed maser emission only to occur,when the temperature in themaser shell is too low to excite radially beamed masers. The radius of the H2O maser shell extends between 6 1014 cm (40 AU) and at least 3 1015 cm (200 AU), with the inner radius probably set by quenching of the maser due to high gas density close to the star.Within theH2Omaser shell, the circumstellar outflow accelerates from v≈9 km s−1 up to the terminal expansion velocity of ≈18 km s−1. The expansion velocity of the circumstellar shell is constant between the outer radius of the water maser shell and the location of the 1612 MHz OH maser shell at 3 1016 cm, showing the absence of strong velocity gradients at large radial distances.