  CONTEXT: Water vapour maser emission from evolved oxygen-rich stars remains
poorly understood. Additional observations, including polarisation studies and
simultaneous observation of different maser transitions may ultimately lead to
greater insight. AIMS: We have aimed to elucidate the nature and structure of
the VY CMa water vapour masers in part by observationally testing a theoretical
prediction of the relative strengths of the 620.701 GHz and the 22.235 GHz
maser components of ortho water vapour. METHODS: In its high-resolution mode
(HRS) the Herschel Heterodyne Instrument for the Infrared (HIFI) offers a
frequency resolution of 0.125 MHz, corresponding to a line-of-sight velocity of
0.06 km/s, which we employed to obtain the strength and linear polarisation of
maser spikes in the spectrum of VY CMa at 620.701 GHz. Simultaneous ground
based observations of the 22.235 GHz maser with the Max-Planck-Institut f\"ur
Radioastronomie 100-meter telescope at Effelsberg, provided a ratio of 620.701
GHz to 22.235 GHz emission. RESULTS:We report the first astronomical detection
to date of water vapour maser emission at 620.701 GHz. In VY CMa both the
620.701 and the 22.235 GHz polarisation are weak. At 620.701 GHz the maser
peaks are superposed on what appears to be a broad emission component, jointly
ejected asymmetrically from the star. We observed the 620.701 GHz emission at
two epochs 21 days apart, both to measure the potential direction of linearly
polarised maser components and to obtain a measure of the longevity of these
components. Although we do not detect significant polarisation levels in the
core of the line, they rise up to approximately 6% in its wings.
