  This paper examines line profile evolution due to the linear expansion of
circumstellar material obsverved during a microlensing event. This work extends
our previous papers on emission line profile evolution from radial and
azimuthal flow during point mass lens events and fold caustic crossings. Both
''flavours'' of microlensing were shown to provide effective diagnostics of
bulk motion in circumstellar envelopes. In this work a different genre of flow
is studied, namely linear homologous expansion, for both point mass lenses and
fold caustic crossings. Linear expansion is of particular relevance to the
effects of microlensing on supernovae at cosmological distances. We derive line
profiles and equivalent widths for the illustrative cases of pure resonance and
pure recombination lines, modelled under the Sobolev approximation. The
efficacy of microlensing as a diagnostic probe of the stellar environs is
demonstrated and discussed.

Astrophysics