MODE SWITCHING OF THE WATER MASER IN OH-39.7+1.5

Monitoring of the H2O maser in OH 39.7+1.5 as well as spatial interfer ometry show clear evidence that the maser switches occasionally its be aming direction between radial and tangential paths in step with the s tellar pulsation cycle. Outside the minimum, several radially beamed m aser features are present, which are absent (less than or equal to 20 mJy) during the minimum. Competition between gain paths causes tangent ially beamed maser emission only to occur, when the temperature in the maser shell is too law to excite radially beamed masers. The radius o f the H2O maser shell extends between 6 10(14) cm (40 AU) and at least 3 10(15) cm (200 AU), with the inner radius probably set by quenching of the maser due to high gas density close to the star. Within the H2 O maser shell, the circumstellar outflow accelerates from v approximat e to 9 km s(-1) up to the terminal expansion velocity of approximate t o 18 km s(-1). The expansion velocity of the circumstellar shell is co nstant between the outer radius of the water maser shell and the locat ion of the 1612 MHz OH maser shell at 3 10(16) cm, showing the absence of strong velocity gradients at large radial distances.