  We have observed SN1998S which exploded in NGC3877, with the UES at the WHT
and with the E230M echelle of STIS aboard HST. Both data sets were obtained at
two seperate epochs. From our own Galaxy we detect interstellar absorption
lines of CaII, FeII, MgI, and probably MnII from the edge of the HVC Complex M.
We derive gas-phase abundances which are very similar to warm disk clouds in
the local ISM, which we believe argues against the HVC material having an
extragalactic origin. At the velocity of NGC3877 we detect interstellar MgI,
MgII, MnII, CaII, & NaI. Surprisingly, one component is seen to increase by a
factor of ~1 dex in N(NaI) and N(MgI) between the two epochs over which the
data were taken. Unusually, our data also show narrow Balmer, HeI, and
metastable FeII P-Cygni profiles, with a narrow absorption component
superimposed on the bottom of the profile's absorption trough. Both the broad
and narrow components of the optical lines are seen to increase substantially
in strength between the two epochs. Most of the low-ionization absorption can
be understood in terms of gas co-rotating with the disk of NGC 3877, providing
the SN is at the back of an HI disk with a similar thickness to that of our own
Galaxy. However, the variable absorption components, and the classic P-Cygni
emission profiles, most likely arise in slow-moving circumstellar outflows
originating from the red supergiant progenitor of SN1998S. [Abridged.]
