  Oxygen-rich Asymptotic Giant Branch (AGB) stars can be intense emitters of
SiO ($v$=1 and 2, J=1$\rightarrow$0) and H$_2$O maser lines at 43 and 22 GHz,
respectively. VLBI observations of the maser emission provide a unique tool to
probe the innermost layers of the circumstellar envelopes in AGB stars.
Nevertheless, the difficulties in achieving astrometrically aligned \water\ and
$v$=1 and $v$=2 SiO maser maps have traditionally limited the physical
constraints that can be placed on the SiO maser pumping mechanism. We present
phase referenced simultaneous spectral-line VLBI images for the SiO $v$=1 and
$v$=2, J=1$\rightarrow$0, and H$_2$O maser emission around the AGB star R\,LMi,
obtained from the Korean VLBI Network (KVN). The simultaneous multi-channel
receivers of the KVN offer great possibilities for astrometry in the frequency
domain. With this facility we have produced images with bona-fide absolute
astrometric registration between high frequency maser transitions of different
species to provide the positions of the \water\ maser emission, and the centre
of the SiO maser emission, and hence reducing the uncertainty in the proper
motion for R\,LMi by an order of magnitude over that from Hipparcos. This is
the first successful demonstration of source frequency phase referencing for
mm-VLBI spectral-line observations and also where the ratio between the
frequencies is not an integer.
