  We report on the first far-UV observations of the asynchronous polar BY Cam
made by the Far-Ultraviolet Spectroscopic Explorer (FUSE). The source is known
to exhibit the most extreme NV/CIV emission resonance line ratio observed among
polars. The FUSE observations reveal a OVI resonance line weaker than in the
prototype of polars, AM Her, with the absence of a detectable narrow component.
The OVI broad line is detected with an equivalent width of the same order as in
AM Her, the blueward doublet component is clearly present but the redward
component is strongly affected by H2 absorption. The presence of a strong NIII
line and weak CIII lines also confirms the peculiar CNO line flux. We compare
the resonance CNO line intensities with the predictions of the CLOUDY plasma
code coupled to a geometrical model of the accretion column. Varying the
temperature and/or intensity of the ionising spectrum is unable to reproduce
the observed broad line ratios. A solution is obtained by significantly
altering the element abundances with a strong depletion of C, overabundance of
N and a weak underabundance of O. This confirms previous suggestions of
non-solar abundances which may result from redistribution in the accreted
material following nova outbursts and/or the secondary nuclear evolution. A
very significant H2 absorption is observed in front of the source, a possible
indication for either the existence of a dense interstellar cloud or of
circumstellar material.
